Hi WTem, welcome to the monastery. As with all things Perl, there is more than one way to do this. As long as you have at least Perl 5.14 (which added the /r regex modifier), this will work too and wont touch the original $interface value:

my ($ip,$interface) = ('192.168.1.1', 'POS1/0/0');
my $filename = sprintf '%s--%s.txt', $ip, $interface =~ s#/##gr;
print $filename, "\n";
[download]

192.168.1.1--POS100.txt