if ($_ != /<exm>/g)

You need to go back and have another read of perlop.

You're saying "if $_ isn't numerically equal to the value returned when you try to match /<exm>/ against $_". What you're trying to say is "if $_ doesn't match /<exm>/". That's written as:

if ($_ !~ /<exm>/)
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or (perhaps easier to understand)

if (!/<exm>/)
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When all else fails, it's always worth reading the documentation for the syntax you're trying to use :-)