The regex you are using does not anchor to the end.
use strict;
use warnings;
use Test::More tests => 4;
my $tomatch = 'GigabitEthernet1/0/48';
my $notmatch = 'GigabitEthernet1/0/49';
my $yourgo = qr/(^gi.*)([1-8])(\/)(0)(\/)([1-9]|[1-3][0-9]|4[0-8])/i;
my $mygo = qr/(^gi.*)([1-8])(\/)(0)(\/)([1-9]|[1-3][0-9]|4[0-8])$/i;
like ($tomatch, $yourgo, "$tomatch matches");
unlike ($notmatch, $yourgo, "$notmatch does not match");
like ($tomatch, $mygo, "$tomatch matches");
unlike ($notmatch, $mygo, "$notmatch does not match");
There's other things you could clean up in the regex and your stipulation of not using a numerical comparison requires justification but this should put you on the right road at least.
