How are you calling the convert() routine? You're declaring a no-arguments prototype (i.e. the empty parentheses), but are then assigning my @array = @_; (presumed to be arguments) at the beginning of the routine. This doesn't make much sense. Similarly, printing something after the return ... :)

Update: BTW, you could also use unpack to simplify the function, e.g. something like

my @mac_addrs = ("0015FAA3F03A", "0015FAA3F03B", "0015FAA3F03C");

sub convert {
    return map { "AP" . join ".", unpack "(A4)*", lc $_ } @_;
}

print "$_\n" for convert(@mac_addrs);
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Output:

AP0015.faa3.f03a
AP0015.faa3.f03b
AP0015.faa3.f03c
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