use warnings;
use strict;

my $file = 'input.txt';

if (@ARGV){
    if (! -e $ARGV[0]){
        warn "$ARGV[0] is not a valid file, using default\n";
    }
    else {
        $file = $ARGV[0];
    }
}

open my $fh, '<', $file or die $!;

while (<$fh>){
    print;
}