#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

my %fastas = (
	'ENSG0000017672' => 'eureka',
);

my $gene_name = 'ENSG0000017672';

print 'test1: ', $fastas{ENSG0000017672}, "\n";
print 'test2: ', $fastas{$gene_name},     "\n";


local $Data::Dumper::Useqq = 1;
print Data::Dumper->Dump([$gene_name], ['gene_name']);

##</code><code>##

use Data::Dumper;

[...]

if (exists $fastas{$gene_name}) {
 $sequence = $fastas{$gene_name};
}
else {
  die Data::Dumper->Dump([$gene_name, \%fastas], ['gene_name', 'fastas']);
}

##</code><code>##

use Data::Dumper;

[...]

exists $fastas{$gene_name}
  or die Data::Dumper->Dump([$gene_name, \%fastas], ['gene_name', 'fastas']);

$sequence = $fastas{$gene_name};