Just another possible alternatives. If what you want to test is a filehandle size you can use Perl's -s function.

my $q = new CGI;
my $filehandle = $q->upload("file");

my $fsize = -s $filehandle;

my $fsize_limit = 1000;

# Then you use conditional to check them:

if ($fsize > $fsize_limit) {

   # do whatever you want

}
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If it the file is tied to a variable you can simply use length function:

# let's say you have a data captured in textrea
      textarea(
                -name    => 'sequence',
                -rows    => 10,
                -columns => 50,
                -wrap    => 'physical'
            ),br

# then you would need to store it into a variable

my $seq_var = param('sequence');

# Size of the variable is captured this way:

my $fsize = length($seq_var);

if ($fsize > $fsize_limit) {

 # Do whatever you want
}
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