#!/usr/bin/perl
use strict;
use warnings;
use IO::Prompt::Hooked;
my %opts = (
    message  => 'Enter namen of input fasta file',
    default  => 'chr.fa',
    validate => sub {-e $_[0]},
    error    => 'File does not exist', 
);

open my $FILE, '<',  prompt(%opts);
open my $out, '>', 'output.txt';

...;