May be a little bit of an overkill, but does what you say you want ;-)

my $reg1 = qr{((?:(?<=<)ab(?=>))|(?:(?<!<)ab(?!>)))};
while(<DATA>) {
    chomp;
    print $_, "\t", m/$reg1/?"yes $1":"no", "\n";
}
__DATA__
<ab
ab>
<>
<ab>
ab
__END__
Output:
<ab     no
ab>     no
<>      no
<ab>    yes ab
ab      yes ab
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