I am going to weigh in here a little late.

It seems to me that all the bother of setting up a while loop is unnecessary if all you're doing is skipping the first line of the file, reading the second, and exiting. I might write such a sub like this:

sub check_field {
    open my $fh, "<", shift or die "Bleah!\n$!";
    <$fh>;    # Skip the unwanted line.
    my @fields = split /;/, <$fh>;
    close $fh;
    die "Ick!\n" unless @fields == 4;
    return( ($fields[2] =~ /^\d+$/) ? 1 : 0 );
}
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It's not really a matter of golf, I just like the idea that if we are only reading the first two lines from a file, listing <$fh> twice instead of breaking out of a while loop after the second line is somehow preferable.

Also, I think your goal is to return true if $fields[2] matches only digits. I've used the ternary operator to ensure that undef never gets returned.