use strict;
use warnings;
sub getHash
{
  my %hash;
  for(my $i=0;$i<4;$i++)
  {
    my $key = <STDIN>;
    chomp($key);
    my $val = <STDIN>;
    chomp($val);

    if ( !defined $hash{$key} ) #first time
    {
        $hash{$key} = $val;
    }
    else
    {
        $hash{key} = ref($hash{key}) ? [ @{$hash{key}}, $val ] : [ $ha
+sh{key}, $val ];
    }
  }
}
[download]

This leaves you however with the ambuigity of deciding wether there is an arrayref or a scalar under the keys of your hash in any code that processes the hash later on. It might be simpler to always use an arrayref, even when there's only a sngle element.

use strict;
use warnings;
sub getHash
{
  my %hash;
  for(my $i=0;$i<4;$i++)
  {
    my $key = <STDIN>;
    chomp($key);
    my $val = <STDIN>;
    chomp($val);

    push @{$hash{key}}, $val;
  }
}
[download]