A question mark is a special regex character that means match the previous character 0 or 1 time:

use strict;
use warnings;
use 5.010;
 
my $location = 'data';
my @fnames = ('/data', '/data/', 'data/');
 
for my $fname (@fnames) {
    if ($fname =~ m{/ $location /?}xms ) {
      say $fname;
    }
}

--output:--
/data
/data/
[download]

But you can use string interpolation in your regex to make your code even clearer:

use strict;
use warnings;
use 5.010;
 
my $location = 'data';
my @fnames = ('/data', '/data/', 'data/');
my $pattern = "/ $location /?";
 
for my $fname (@fnames) {
    if ($fname =~ /$pattern/xms ) {
      say $fname;
    }
}

--output:--
/data
/data/
[download]

You can use the /o flag to signal to perl that the variable won't change and to compile the regex only 'o'nce. That keeps perl from recompiling the regex evertime you try a match.