I think you'd want that RE within an if condition, because otherwise the RE could fail to match and you'd load the previous value of $1 into $var

#!/usr/bin/perl

use warnings;
use strict;

my $str = "http://172.20.37.115:8080/se/1.0/provision/subscribers/1989
+68";

$str =~ /.*\/(\d+)$/;
my $var = $1 || '';

print "$var\n";
# prints 198968

my $str2 = http://172.20.37.115:8080/se/1.0/provision/subscribers/foo"
+;
$str2 =~ /.*\/(\d+)$/;
my $var2 = $1 || '';

print "$var2\n";
#prints 198968
[download]

May be better to do:

use warnings;
use strict;

my $str = "http://172.20.37.115:8080/se/1.0/provision/subscribers/1989
+68";

if( $str =~ /.*\/(\d+)$/ ) {
  my $var = $1 || '';

  print "$var\n";
}
[download]

But I was matching against  (\d*) \z which cannot fail, although it could match an empty string of digits — which can be useful in and of itself!

Update: Interestingly, what is captured | assigned to  $var in a statement like
    my ($var) = $s =~ m{ (\d+) \z }xms;
is not the same as what is captured to | not necessarily the same as the value of $1:

c:\@Work\Perl\monks>perl -wMstrict -le
"'zot' =~ m{ (zot) }xms;
 ;;
 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) = $s =~ m{ (\d+) \z }xms;
   print '$var is ', defined($var) ? '' : 'UN', 'defined';
   print qq{'$s' -> '$var' (\$1 is '$1')};
   }
"
$var is UNdefined
Use of uninitialized value $var in concatenation (.) or string at -e l
+ine 1.
'http://172.20.37.115:8080/se/1.0/pro/subs/198968/' -> '' ($1 is 'zot'
+)
$var is defined
'http://172.20.37.115.8080/se/1.0/pro/123456' -> '123456' ($1 is '1234
+56')
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This warrants a bit more research!

---

Give a man a fish:  <%-(-(-(-<

Interestingly, what is captured to $var in a statement like

    my ($var) = $s =~ m{ (\d+) \z }xms;
[download]

is not the same as what is captured to $1:

Isn't that just because the RE didn't match the first time through the loop, so $1 still held 'zot' ?

If your RE did match the first time through, $1 would have been reset and would therefore match $var ...

Anomalous, sorry, my comment was not in response to yours, but to the one above it by Stevieb :-)

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]