print "$_\n" for transposition_list('thisisatest'); 

sub transposition_list {
    my $string = shift; 
    my @transpositions; 

    for my $i (0 .. length($string)-2) { 
        (my $tmp = $string) =~ s/(.{$i})(.)(.)/$1$3$2/;
        push @transpositions, $tmp; 
    }

    return @transpositions; 
}