You can treat constants as subroutines (which they are). In your case, the statement should look something like this: 
$s1 =~ s/&TXT_EXTENSION/&MIF_EXTENSION/e;

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Note the 'e' quantifier after the regex -- this tells the engine to evaluate the substitute before the substitution.

Update: this code is incorrect, thanks to 3dan for pointing this out.

In reply to Re: use of constants in regex substitutions? by dmitri
in thread use of constants in regex substitutions? by Anonymous Monk

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