Just as an aside, according to perlvar (about $&):
The use of this variable anywhere in a program imposes a considerable performance penalty on all regular expression matches.


It might be 'better' to do: 
s/(?<=[a-zA-Z])([12])/ $1 == 1 ? 2 : 1 /eg;

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(although I have to say I'm not sure why you'd need the positive look-behind instead of just a normal match?)

In reply to Re^2: replace a digit with another digit by Transient
in thread replace a digit with another digit by kevyt

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