Just for fun, here's your code reworked to be a little more idiomatic:

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my @unsortedData = ( 5, 4, 7, 2, 1, 3, 6, 9, 8, 10 );
print "@unsortedData\n";

mergeSort( \@unsortedData );

print "@unsortedData\n";

sub mergeSort {
    my( $data ) = @_;
    return $data if @$data < 2;  # if there is less then two items it'
+s already sorted!!

    my $middle = @$data / 2;

    my $leftSide  = [ @$data[ 0 .. $middle -1 ] ];
    my $rightSide = [ @$data[ $middle .. $#{ $data } ] ];
    mergeSort( $leftSide );
    mergeSort( $rightSide );
    merge( $leftSide, $rightSide, $data );
    return $data;
}

sub merge {
    my ( $groupA, $groupB, $dataRef ) = @_;
    my( $i, $j, $k ) = (0)x3;

    @$dataRef[$k++] =
        $groupA->[$i] <= $groupB->[$j] ? $groupA->[$i++] : $groupB->[$
+j++]
        while $i < @$groupA && $j < @$groupB;

    @$dataRef[$k++] = $groupA->[$i++] while $i < @$groupA;
    @$dataRef[$k++] = $groupB->[$j++] while $j < @$groupB;
}
[download]

It also demonstrates something to be aware of; namely, it doesn't use a variable: $sortedData!