The replacement part of a regex acts like a double quoted string as far as variable interpolation is concerned, so this should work (untested): 
s/([a-m])([n-z])/${1}5$2/g;
#                ^^^^

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In reply to Re: Inserting numbers between parenthese matches in regexp by Joost
in thread Inserting numbers between parenthese matches in regexp by Melroch

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