It's unlikely but not beyond the realms of possibility to have more than ten interfaces so it would be safer to match for one or more (+) digits at the end of the string. This gives the wrong result:-

 $ perl -Mstrict -Mwarnings -E '
my $interface = q{bge12};
$interface =~ s{[0-9]$}{:$&};
say $interface;'
bge1:2
$
[download]

This corrects the problem. I also prefer to use the \d special escape for digits and a look-ahead to save having to mention what was matched in the replacement part:-

 $ perl -Mstrict -Mwarnings -E '
my $interface = q{bge12};
$interface =~ s{(?=\d+$)}{:};
say $interface;'
bge:12
$
[download]

I hope this is of interest.