The easiest is to change the call to open to something more suitable, like opening a scalar reference ("in-memory file"), as described in perlopentut.

If you absolutely don't want to modify the call to open(), you can replace open() with your own open() sub, which intercepts the opening process, and then returns an in-memory file handle instead. See perlsub for details (search for CORE::)

Another option is to use something like ptrace (not very portable) to intercept system calls to fopen. But that's probably really overkill.

#!/usr/bin/perl

use strict;
use warnings;
use IO::String;

my $str = <<EOT;

Four score and seven years ago our fathers
brought forth on this continent, 
a new nation, conceived in Liberty, and dedicated 
to the proposition that all men are created equal.
EOT

my $io = IO::String->new($str);
$io->open('STDOUT');
my @lines = <$io>;
$io->getline;
my $buf = "";
read($io, $buf, 100);
select $io;
print STDOUT "$str\n";
$io->close('STDOUT');
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You can also open a file handle directly against a reference to a HEREDOC as well as a scalar reference.

knoppix@Microknoppix:~$ perl -Mstrict -wE '
> open my $inFH, q{<}, \ <<EOD or die qq{open: <<HEREDOC: $!\n};
> some letters
> this line also 99 has digits
> this does not
> EOD
> 
> while ( <$inFH> )
> {
>     print if m{\d};
> }
> 
> close $inFH or die qq{close: <<HEREDOC: $!\n};'
this line also 99 has digits
knoppix@Microknoppix:~$
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I hope this is helpful.

%INC is for modules, not just any files

Use 3 argument open and here-docs, example

#!/usr/bin/perl --
use strict;
use warnings;
use autodie;    # dies if open/close... fail

Main(@ARGV);
exit(0);

sub Main {
    Demo();
}

sub Demo {
    my ( $Input, $Input2, $WantedOutput ) = DemoData();
    NotDemoMeaningfulName( $Input, $Input2, \my $Output );
    ...
}

sub DemoData {
    my $One = <<'__One__';
Optometrists and Opticians (simple) Regulations, Cap. 500A, RG 2
__One__

    my $Two = <<'__Two__';
OPTOMETRISTS AND OPTICIANS (SIMPLE) REGULATIONS, Cap. 500A, Regulation
+ 2
__Two__

    my $Three = <<'__Two__';
{RG 2}{Regulation 2}
__Two__

    return \$One, \$Two, $Three;
} ## end sub DemoData

sub NotDemoMeaningfulName {
    my ( $inputOne, $inputTwo, $outputFile ) = @_;
    open my ($inOne), '<', $inputOne;
    open my ($inTwo), '<', $inputTwo;
    open my ($outFh), '>', $outputFile;
...
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