echo abcd |  perl -pe "s/(ab)(cd)?/defined($2)?12:''/e"

Backslash found where operator expected at -e line 1, near "1"
(Missing operator before ?)
syntax error at -e line 1, near "1"
Execution of -e aborted due to compilation errors.

echo abcd | perl -pe's/(ab)(cd)?/defined($2)?$1.$2:""/e'

echo abcd | perl -pe's/(ab(cd)?)/defined($2)?$1:""/e'

echo abcd | perl -pe's/ab(?!cd)//'