c:\@Work\Perl\monks>perl -wMstrict -le
"my @flist = qw(
   1st-device-CA_Eth.0/1
   2nd-device_TX_Gig.1/1
   3rd_device_MO_Gig.1/1
   );
 ;;
 for my $s (@flist) {
   print qq{'$s'};
   $s =~ s{ _ (?= [^_]* \z) }{,}xms;
   print qq{'$s'};
   print '';
   }
"
'1st-device-CA_Eth.0/1'
'1st-device-CA,Eth.0/1'

'2nd-device_TX_Gig.1/1'
'2nd-device_TX,Gig.1/1'

'3rd_device_MO_Gig.1/1'
'3rd_device_MO,Gig.1/1'