G'day loris,

The reason you're getting that error is because the heredoc (in your module) is being interpolated.

In your module, you've probably either coded <<"PROLOG" or <<PROLOG.

See perlop: Quote-Like Operators for an explanation of how explicit single- and double-quoted terminating strings (i.e. PROLOG in your example) are interpolated; also note that the absence of explicit quotes assumes double-quotes.

There are a variety of hoops you have to jump through to get similar code working in a one-liner. These have mostly been pointed out in various replies. You could embed newlines in your one-liner and do something like this:

$ perl -Mstrict -Mwarnings -e 'package test; my $text = <<\TEST;
test@heredoc 
TEST
print $text'
test@heredoc
[download]

That's not using the module as such; just running the code in a package other than main. You're also using a different (albeit equivalent) method of quoting the terminator (i.e. <<\TEST). What you'd really want is something closer to this:

$ perl -Mstrict -Mwarnings -e 'package test; my $text = <<\TEST;
test@heredoc
TEST
sub test_print { print $text }
package main;
use parent -norequire => "test";
test::test_print()'
test@heredoc
[download]

That's a lot of effort; it's not really comparing apples with apples; and, given you could have copied most of the code you already had, probably takes a similar amount of time as coding this test script:

#!/usr/bin/env perl

use strict;
use warnings;

use pm_example_module;

pm_example_module::test_print();
[download]

and this test module:

package pm_example_module;

use strict;
use warnings;

my %text_blocks;
$text_blocks{en}{prolog} =<<'PROLOG';
You have received this email because something happened.

Please send feedback and suggestions to tldr@nohelp.org.

PROLOG

sub test_print {
    print $text_blocks{en}{prolog};
} 

1;
[download]

Running that script produces:

You have received this email because something happened.

Please send feedback and suggestions to tldr@nohelp.org.
[download]

Changing <<'PROLOG' to either <<"PROLOG" or <<PROLOG, produces (in both cases):

Possible unintended interpolation of @nohelp in string at pm_example_m
+odule.pm line 8.
Global symbol "@nohelp" requires explicit package name at pm_example_m
+odule.pm line 8.
Compilation failed in require at ./pm_example.pl line 6.
BEGIN failed--compilation aborted at ./pm_example.pl line 6.
[download]

You now have a valid test which reproduces what you were attempting to test and, I suspect, would have taken less time than writing and posting your OP.