Its hard to tell what you want since you didn't tell us specificaly. It appears however than you want to put the 0 on the front if there are only for digits? You could match for 5 and reverse your if statment or do the following.

my $test = "2123";
if($test=~/^\d{4}$/)
{    
          print "<td height=20 class=grtxt>0$test</td>\n";
}
else
{          
          print "<td height=20 class=grtxt>$test</td>\n";          
}
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I removed the + since plus means 1 or more and your {4} means four. I then added ^ to the start and $ to the end, that means the regex will only match if it matches the whoel line. So it will only be happy if the string is 4 digits exactly. I'm still not sure this is the best solution since you repeated your HTML without cause. It would be far better to modify $test or have a second variable to hold the output string.

my $test = "2523";
$test = "0" . $test if($test=~/^\d{4}$/);
print "<td height=20 class=grtxt>$test</td>\n";
[download]