It may be that what you want is:

    while (defined (my $line = <WORDS>)) {
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which avoids issues with values for $line that evaluate to false in a boolean context.

If that is not your problem then a better example would include some data like this:

use strict;
use warnings;

while (<DATA>) {
    chomp;
    my @words = split /\s+/;
    
    while (@words) {
        $_ = shift @words;
        next if ! defined $_;
        next unless /a/ && /e/ && /i/  && /o/ && /u/;
        print "Matched: $_\n";
    }
}

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aeiou
0

nothing here
aeiou again
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