You finction can be written more simply as

sub getFoo{
    my ($x,$y) = @_;
    $x . '_' . $y . '_testme.xml';
}
[download]

or just:

sub getFoo{
    sprintf '%s_%s_testme.xml', @_;
}
[download]

Notice that the package global names $a and $b are not used. It's best to preserve them for sort.

After Compline,
Zaxo