use strict;

my $unitslist = join "|", qw(
  1001 1002 1003 1004 1101 1102 1103 1104 1201 1202 1203 1204 1301 1302
  1303 1304 1401 1402 1403 1404 1501 1502 1503 1504 1601 1602 1603 1604
  1701 1702 1703 1704 1801 1802 1803 1804 1901 1902 1903 1904 2001 2002
  2003 2004 2101 2102 2103 2104 2201 2202 2203 2204 2301 2302 2303 2304
  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;