%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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