What is the difference between these two fragments of Perl?

Question

print <<EOF stuff EOF ; print <<EOF; stuff EOF 

Why are you using one above the other?

Answer 1

These two examples apply to the same behavior, but consider whether you want to do something else after printing this block:

 print <<EOF stuff EOF . "more text here"; 

... or maybe you need to check the result of the operation:

 print $unstable_filehandle <<EOF stuff EOF or warn "That filehandle finally disappeared: $!"; 

These examples are far-fetched, but you can see how it can sometimes be useful to be flexible as to what code appears after a block of text.

Answer 2

As tchrist points out, one thing that most people neglect is that the heredoc operator accepts arbitrary Perl code after the termination code.

This means that you can do (possibly) more natural manipulations, such as:

 my $obj = Foo::Bar->new(content => <<EOP)->do_stuff->sprint(); This is my content It from a heredoc. EOP 

One of the consequences is that you can also stack their MUCHs more readably (in my opinion;) than “unfastened” ones:

 -- stacked_heredoc.pl #!/usr/bin/perl use strict; use warnings; print join(<<JOINER, split("\n", <<SOURCE)); ------------------------------ JOINER This is my text this is my other text a third line will get divided! SOURCE 

poems unidentified heredoc ...

 -- unstacked_heredoc.pl #!/usr/bin/perl use strict; use warnings; my $joiner = <<JOINER ------------------------------ JOINER ; my $source = <<SOURCE This is my text this is my other text a third line will get divided! SOURCE ; print join($joiner, split("\n", $source)); 

Answer 3

You must treat heredoc markers just like string literals. Do not delay further punctuation or syntax until their contents. This is misleading and error prone. Here are ten examples taken from real code:

• ($is_a_valid_rfc_822_addr = <<'EOSCARY') =~ s/\n//g;
• $eval = (($Preamble=<<'END1') . $userstuff . <<'END2');
• for my $line (<<"End_of_Property_List" =~ m{ \S .* \S }gx) {
• $cases .= <<"EDQ" if $timeout;
• ($is_a_valid_rfc_822_addr = <<'EOSCARY') =~ s/\n//g;
• eval (($Preamble=<<'END1') . $_[0] . <<'END2');
• @changes = split("\n", <<"EOCHANGES");
• $change .= sprintf(<<"EOP", $in, $out, $in, $out);
• eval "{ package $package; " . <<'EOF' . "}";
• push @args, dequeue('|Q|', <<'END_OF_ASTRAL_MATCH') if $Opt{astral};

See how it works?

Answer 4

FWIW, Perl Best Practices recommends:

 print <<'EOF'; stuff EOF 

Answer 5

Randal Schwartz has a great article here HERE .

There are a few things to remember about documents:

• ; in Perl, it is an operator terminator and is required for all Perl statements (with some exceptions), including document lines here;
• This document is a fun view that really is just a big line;
• This document should have a trailing CR (unlike most other Perl statements);
• You can have arbitrary Perl code either immediately after the opening tag, or after the closing tag. If after the closing tag it should be on a separate line. In any case, the code works on the text of the line here:
• The quotation marks you use in tags affect the string. Double quotes are similar to double quote strings in Perl, single quotes do not interpolate, back quotes are passed to the shell. Quotation marks are not equivalent to double quotation marks;
• TIMTOWTDI, but some are hard to read,

• Quote from perldoc -q "HERE documents" (and perlfaq4 ):

  There should be no space after the <part.

  There (maybe) there should be a semicolon at the end of the [part << ].

  You cannot (easily) have a place in front of a tag.

The two forms that you have are functionally equivalent. Just like { ... } if (blah blah) matches if (blah blah) { ... } . Although the two statements are functionally equivalent, they read differently.

Each of these is equivalent and valid for Perl documents:

 my %data = <<END fred: Fred Flintstone barney: Barney Rubble betty: Betty Rubble wilma: Wilma Flintstone END =~ /(\w+): (.*)/g; 

and

 my %data = <<END =~ /(\w+): (.*)/g; fred: Fred Flintstone barney: Barney Rubble betty: Betty Rubble wilma: Wilma Flintstone END # You must have a CR after the "END". Can't be EOF... 

Both set the %data hash to first=>"full name" for Flintstones. Which would you rather see in the code that came to you?

Note that the second form is the one where gotcha is: There must be text or a space after the ending label, or you can get Can't find string terminator "END" anywhere before EOF I think that’s why you see lonely ; for some people here are docs,

IMHO,; belongs after the first instance of the doc here tag or the code that follows it. It is harder to read if it is after the closing tag. If the closing tag is not in a form similar to an operator modifier or warning or matrix logic. This is just my personal leadership style.