Assuming, as you do, the extraction of a group of digits at the absolute end of a string, the  .*\/ portion of the regex is unnecessary (although it does no harm), as is the subsequent
    my $var = $1 || '';
capture/fixup statement. (Update: But see also the reply of 1nickt below.) An alternative would be:

c:\@Work\Perl\monks>perl -wMstrict -le
"for my $s (
   'http://172.20.37.115:8080/se/1.0/pro/subs/198968',
   'http://172.20.37.115.8080/se/1.0/pro/123456/',
   ) {
   my ($var) = $s =~ m{ (\d*) \z }xms;
   print qq{'$s' -> '$var'};
   }
"
'http://172.20.37.115:8080/se/1.0/pro/subs/198968' -> '198968'
'http://172.20.37.115.8080/se/1.0/pro/123456/' -> ''
[download]

with the regex expression  m{ \d* \z }xmsg (note the added  /g modifier) also working.