I think this might meet your specs:

#!/usr/bin/perl

use strict;
use warnings; 
use Data::Dumper;

my @array1 = (1,2,3); 
my @array2 = (1,2,3,4,5); 

my %hash  = map { $_ => 1 } @array1;
my %hash2 = map { $_ => 1 } @array2; 

print "Before:\n" . Dumper(\%hash) . Dumper(\%hash2);

# remove the unwanted "value" 
my $to_delete = 2;
my $old = delete($hash{$to_delete}); 
my $old2 = delete($hash2{$to_delete}); 

for my $x (keys %hash2) {
  next if exists($hash{$x}); 
  $hash{$x} = 1; 
  delete($hash2{$x});
  last;  # updated -- added this line 
} 

print "After:\n" . Dumper(\%hash) . Dumper(\%hash2);

__OUTPUT__

Before:
$VAR1 = {
          '1' => 1,
          '3' => 1,
          '2' => 1
        };
$VAR1 = {
          '4' => 1,
          '1' => 1,
          '3' => 1,
          '2' => 1,
          '5' => 1
        };
After:
$VAR1 = {
          '4' => 1,
          '1' => 1,
          '3' => 1,
        };
$VAR1 = {
          '1' => 1,
          '3' => 1,
          '5' => 1,
        };
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