You can replace all unit numbers at once if you build your regex pattern out of them. For instance:

use strict;

my $unitslist = join "|", qw(
  1001 1002 1003 1004 1101 1102 1103 1104 1201 1202 1203 1204 1301 130
+2
  1303 1304 1401 1402 1403 1404 1501 1502 1503 1504 1601 1602 1603 160
+4
  1701 1702 1703 1704 1801 1802 1803 1804 1901 1902 1903 1904 2001 200
+2
  2003 2004 2101 2102 2103 2104 2201 2202 2203 2204 2301 2302 2303 230
+4
  2401 2402 2403 2501 2502 2503 2504 2505
);

my $text = "Visit units 1101 or 2202, call us at 555-555-5555\n";

$text =~ s{ \b($unitslist)\b }
          {<a href="apartments.pl?do_what=view&unit=$1"><b>$1</b></a>}
+xg;

print $text;
[download]

Update: Added \b boundary matches to pattern matcher