You could initialise your key/value pairs as an array before transforming into a hash using splice and concatenation in a while loop.

knoppix@Microknoppix:~$ perl -Mstrict -MData::Dumper -e '
> use warnings;
> 
> my @keysAndValues = qw{
>    rat acggghhh
>    mat dhhdhdhdh
>    rat fhhfjfjj
>    rat dggdgdgdg
>    };
> my %hash;
> while ( my( $key, $value ) = splice @keysAndValues, 0, 2 )
> {
>     $hash{ $key } .= $value;
> }
> print Data::Dumper->Dumpxs( [ \ %hash ], [ qw{ *hash } ] );'
%hash = (
          'rat' => 'acggghhhfhhfjfjjdggdgdgdg',
          'mat' => 'dhhdhdhdh'
        );
knoppix@Microknoppix:~$
[download]

I hope this is helpful.