That was me messing with prototypes another topic im still learning in perl. however my code above is directly copy pasted from my real code i changed nothing and i did not just add use strict or use warnings for posting, yet i get nothing like what you got. ill fix the issues, but this leads me to think something is wrong with my enviorment.. EDIT: Oops lol the warnings were scrolling off screen in my terminal i feel dumb. Still get the error though. heres the new code:

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

#protos
# sub mergeSort( \@ );

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

# show the original data to screen
for( @unsortedData ) {
    print "$_  ";
}
print "\n";

my $sortedData = &mergeSort( \@unsortedData );

# show the new data to screen
for( @$sortedData ) {
    print "$_  ";
}
print "\n";

### dereferencing of arrays isnt working in below method

sub mergeSort {
    my ( $data ) = @_;

    if( @$data < 2 ) { return $data; }  # if there is less then two it
+ems it's already sorted!!
    
    use integer;
    #split the array into two parts
    my $middle = @$data / 2; 
    print "middle: $middle\n";
    
    my @leftSide = (); 
    print "LeftSide:\n";
    for( my $i = 0; $i < $middle; $i++ ) {
        $leftSide[$i] = @$data[$i];
        print "$leftSide[$i] ";
    }
    print "\n";
    my @rightSide = (); 
    print "RightSide:\n";
    for( my $i = $middle; $i < @$data; $i++ ) {
        $rightSide[$i - $middle] = @$data[$i];
        print "$rightSide[$i - $middle] ";
    }
    print "\n";
    &mergeSort( @leftSide );
    &mergeSort( @rightSide );
    &merge( @leftSide, @rightSide, $data );
    return $data;
}

sub merge {
    my ( @groupA, @groupB, $dataRef ) = @_;
    my $i = 0; my $j = 0; my $k = 0;
    
    while( $i < @groupA && $j < @groupB ) {
        if( $groupA[$i] <= $groupB[$j] ) {
            @$dataRef[$k] = $groupA[$i];
            $i++;
        }
        else {
            @$dataRef[$k] = $groupB[$j];
            $j++;
        }
        $k++;
    }
    while( $i < @groupA ) {
        @$dataRef[$k] = $groupA[$i];
        $i++;
        $k++;
    }
    while( $j < @groupB ) {
        @$dataRef[$k] = $groupB[$j];
        $j++;
        $k++;
    }
}
[download]

Your problems are pretty much all caused by misuse of prototypes. Just get rid of them. There is rarely a good reason to give Perl subs prototypes.

use Moops; class Cow :rw { has name => (default => 'Ermintrude') }; say Cow->new->name

Compare this code (which works) with your version and ask questions about any differences you do not understand:

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

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

# show the original data to screen
for( @unsortedData ) {
    print "$_  ";
}
print "\n";

my $sortedData = mergeSort( \@unsortedData );

# show the new data to screen
for( @$sortedData ) {
    print "$_  ";
}
print "\n";

### dereferencing of arrays isnt working in below method

sub mergeSort {
    my ( $data ) = @_;

    if( @$data < 2 ) { return $data; }  # if there is less then two it
+ems it's already sorted!!

    use integer;
    #split the array into two parts
    my $middle = @$data / 2;
    print "middle: $middle\n";

    my @leftSide = ();
    print "LeftSide:\n";
    for( my $i = 0; $i < $middle; $i++ ) {
        $leftSide[$i] = @$data[$i];
        print "$leftSide[$i] ";
    }
    print "\n";
    my @rightSide = ();
    print "RightSide:\n";
    for( my $i = $middle; $i < @$data; $i++ ) {
        $rightSide[$i - $middle] = @$data[$i];
        print "$rightSide[$i - $middle] ";
    }
    print "\n";
    mergeSort( \@leftSide );
    mergeSort( \@rightSide );
    merge( \@leftSide, \@rightSide, $data );
    return $data;
}

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

    while( $i < @$groupA && $j < @$groupB ) {
        if( $groupA->[$i] <= $groupB->[$j] ) {
            @$dataRef[$k] = $groupA->[$i];
            $i++;
        }
        else {
            @$dataRef[$k] = $groupB->[$j];
            $j++;
        }
        $k++;
    }
    while( $i < @$groupA ) {
        @$dataRef[$k] = $groupA->[$i];
        $i++;
        $k++;
    }
    while( $j < @$groupB ) {
        @$dataRef[$k] = $groupB->[$j];
        $j++;
        $k++;
    }
}
[download]

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i understand that you made the merge method take references but i am confused to why. could you explain that to me?