c:\@Work\Perl\monks>perl -wMstrict -le
"for my $string (qw(
   50.10-d557_5  50.10-d557_8  50.10-d557_20  50.10-d557_123
   )) {
   my ($value) = $string =~ m{ _ (\d+) \z }xms;
   print qq{'$string' -> '$value'};
   }
"
'50.10-d557_5' -> '5'
'50.10-d557_8' -> '8'
'50.10-d557_20' -> '20'
'50.10-d557_123' -> '123'