I would write that as: 
"abcd" =~ /(..) (?{push @pairs, $1}) (*FAIL)/xs;
say "@pairs";
__END__
ab bc cd

[download]
Note the absence of zero-width matches; together with the (*FAIL), this causes the regexp engine to restart with the next character. The zero-width technique relies on the engine making an exception: a second successive zero-width match is rejected -- however, in order to determine the second match has zero length, it has to execute the block.

In reply to Re: Regex: Overlapping Matches: Double Execution of Eval-ed Code by JavaFan
in thread Regex: Overlapping Matches: Double Execution of Eval-ed Code by AnomalousMonk

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