Further to davorq's most apposite observation, even if $_ had been defined, the $_ =~ is redundant since $_ =~ /RE/ and /RE/ are exactly equivalent.

You might also want to use the standard strictures and warnings.

I'm guessing something like this might be a little closer to what you want (to check that, for each tag in a list of tags, the tag body can be found in a named array of valid tag names - or some such) ...

use strict;
use warnings;
.
.
sub validate_tag {
    # Assume tags passed as arg list
    while("@_" =~ /<(.+?)>/g) {
        print "Invalid tag $1 found" unless grep /(:?$1)/, @{$ARGV[1]}
+;
    }
}
.
.
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