What do you want it to be? Grep loops over all of the input, so the only thing that makes sense (to me) is the last match:

my $last_match;
if ( grep { /^tag\: (.*)$/ and $last_match = $1 and 1} <FILE> ) {
        print "Found tag: $last_match\n";
}
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more likely you want all matches:

while (<FILE>) {
   next unless /^tag\: (.*)$/;
   print "Found tag: $1\n";
}
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edit: or even

print map /^tag\: (.*)$/ ? "Found tag: $1\n" : (), <FILE>;
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Hope this helps,
Joost.