my @strings = qw(
  /home/usernames/doejohnwilson
  /home/usernames/doejanemary
);

my $common = $strings[0] ^ $strings[1]; # every \x00 means the chars are identical
my $common_length = 0;
if ($common =~ /^(\x00+)/) {
    $common_length = length $1;
};
print substr $strings[0], 0, $common_length;