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use warnings;
use strict;
...
$text =~ s{ ($digits_4) }
          {<a href="script.pl?do=view&unit=$1"><b>$1</b></a>}xmsg;
print qq{[[$text]] \n};

c:\@Work\Perl\monks\htmanning>perl s_4_digits_1.pl
[[foo 1234
...
 <a href="script.pl?do=view&unit=1234"><b>1234</b></a> bar
<a href="script.pl?do=view&unit=1234"><b>1234</b></a>
 1 22 333 55555 666666]]