use feature qw( say );

use Algorithm::Loops qw( NestedLoops );

sub someFunction {
   my ($s, $subs) = @_;
   my @loops = map [ $_ ], split //, $s;
   for (keys(%$subs)) {
      push @{ $loops[$_-1] }, split //, $subs->{$_};
   }

   return NestedLoops(\@loops, sub { join('', @_) });
}

my $s = 'fred';
my %subs = (
    2 => 'xy',
    3 => 'ab',
);

say for someFunction($s, \%subs);
[download]

Algorithm::Loops

Updated: Fixed syntax errors in the parts I copied from the OP.