How about this?

#! /usr/local/bin/perl -w

use strict;

sub  forward {$a cmp $b} # removed prototype
sub backward {$b cmp $a} # removed prototype

my @sorter = qw/forward backward/;

my @list = qw(kholsky stencil dnubietna barkhausen
    manganese fairing fleische flake schlozhauer gascoigne);

my $offset = shift || 0;

my $subname= $sorter[$offset];
print "$_\n" for sort $subname @list;
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Simply supply the sort routine's name.

Update: Thanks to kwaping for noticing that this doesn't work. In fact I oversaw the word "forward" inside the output. To fix it I needed the 3 changes shown above. Prototype must be removed because If the subroutine's prototype is "($$)", the elements to be compared are passed by reference in @_, as for a normal subroutine.. The aditional $subname is needed because: SUBNAME may be a scalar variable name (unsubscripted), in which case the value provides the name of (or a reference to) the actual subroutine to use.