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my $test = "2123";
if($test=~/^\d{4}$/)
...
{          
          print "<td height=20 class=grtxt>$test</td>\n";          
}

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