From perlop (under "Quote-like operators"):

The empty pattern //

If the PATTERN evaluates to the empty string, the last successfully matched regular expression is used instead. In this case, only the g and c flags on the empty pattern is honoured - the other flags are taken from the original pattern. If no match has previously succeeded, this will (silently) act instead as a genuine empty pattern (which will always match).

This is an odd "feature" of Perl, and it's very easy to get bitten by it in m/$pattern/. It's happened to me several times. Perhaps a good solution is m/(?:$pattern)/ or m/(?:)$pattern/ or something similar, so that the pattern is never empty.

Update:

$ perl -le 'print 1 if "foo" =~ /o/; print 2 if "asdf" =~ //'
1

$ perl -le 'print 1 if "foo" =~ /z/; print 2 if "asdf" =~ //'
2

$ perl -le 'print 1 if "foo" =~ /o/; print 2 if "asdf" =~ /(?:)/'
1
2
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