Yes exactly. If the regex doesn't matchm $fields2 will be undef. Also, I think you need to put parenthesis around your ( split /;/ ) so that the context is the same (list context):

use strict;
my @fields;
my $RS = ' ';

my $rv = &do_this;

print $rv;



sub do_this {

while(<DATA>){
    
    next unless $. == 2;
    @fields = ( split /$RS/, $_ ), last if $. == 2;
@fields = ( split /$RS/, $_ ), last if $. == 2;

}

return $fields[2] =~ /\d+/;

}


__DATA__
This is line one.
I found 12 on line two.
And This was found on line three.
[download]

Change 12 on line to some text and try removing the parens around the split... and you will see the diffs.
JamesNC