Is this a stealth CGI question? Including this:

my $checkthree = param('check');
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would tend to indicate that it is.

You really should tell people, it is often highly relevant, albeit I think not in this case.

You really, really should start your scripts by enabling warnings and strictures. It may seem like a lot of work, but in the long run it will pay you back in saved time a thousandfold.

use strict;
use warnings;

my $checkthree = param('check');
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You shouldn't use the filehandle DATA. DATA is a special filehandle for reading in data embedded in your script after the __END__ tag.

open IN, $data or die "Cannot open $data for reading:$!\n";
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There seems to be no need to read the data file into an array since you are only looping through it once. Use a while loop instead. Don't define variables that you don't use when splitting up the line.

while (my $line =<DATA>)
{
    my (undef,undef,$three,$four,undef) = split "\t",$line;
    if ($three eq $checkthree) {print "$three found before $four in li
+ne $.\n"; last;}    
}
close DATA;
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--
Regards,
Helgi Briem
helgi AT decode DOT is