A positive match would be:

[<>/]   # char that is "<", ">" or "/"
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Add ^ for a negative match:

[^<>/]   # char that isn't "<", ">" or "/"
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In addition to your pattern being bad, your tests strings are bad (<\Comment>).

use Test::More tests => 2;

$comment = '<Comment>foo bar</Comment>';
ok( $comment =~ m{<Comment>[^<>/]*</Comment>}, 'Detect no specials');

$comment = '<Comment>foo < bar</Comment>';
ok( $comment !~ m{<Comment>[^<>/]*</Comment>}, 'Detect <');
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$ perl -MTest::Harness -e'runtests @ARGV' 808525.t
808525.t .. ok
All tests successful.
Files=1, Tests=2,  0 wallclock secs
Result: PASS
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