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");
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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.