Ugly but works (for some definition of 'works'): 
$line =~ s/BA/XXXXsome magic sequenceXXXX/g;
$line =~ s/A/Z/g;
$line =~ s/XXXXsome magic sequenceXXXX/BA/g;

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In reply to Re: look behind in REs by jbert
in thread look behind in REs by tepas

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