What if "this dog" appears? The pattern will match "this" and move on to the next line. You would need to to a global (/g) match on the line, and then sift through what you found.

One interesting way to use the superterm approach is to remove each term you find from it as you go: 

my @file = <DATA>;
my @array = ('file', 'this', 'dog', 'forward');
my $superterm = join '|', @array;
my @stringsfound;
my %guard;
for (@file) {
    while ($superterm ne '' and /($superterm)/gi) {
        print "Found <$1>\n"; # To watch it work
        push @stringsfound, $1;
        $guard{$1} = undef;
        $superterm = join '|', grep {!exists $guard{$_}} @array;
    }
}
print map "$_\n", @stringsfound;
__DATA__
No one could find this dog dog dog
a young boy came forward to claim this file
but his file was a mile long

[download]
Caution: Contents may have been coded under pressure.

In reply to Re^2: Match a string only once in a file by Roy Johnson
in thread Match a string only once in a file by newbie2perl

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