my ($num)= $seqnum=~/^(\d+)/;

This would extract the first string of digits inside the string $seqnum. I'm not sure whether this is what the OP wanted. Giving the call to the write-function in the posting, I rather think the OP would like to catch those cases where the string does not evaluate to an integral number. I think one portable solution would be

use Scalar::Util qw(looks_like_number);
if(looks_like_number($seqnum) && int($seqnum)==$seqnum) {
  # $seqnum is arithmetically an integral number
}
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