I have a list of files:
file1_1.pdf file1_2.pdf file1_3.pdf
file1_a.pdf file1_b.pdf file1_c.pdf
You could try something like this:
rename 's/_\K(\d+)(?=.pdf$)/chr ((ord "a") + $1 - 1)/e' *.pdf
eoption to the substitution operator is used to convert the digit to a lowercase character, see perlop for more details on the
\K, see perlre for more details.
(?=pat)zero-width positive lookahead assertion, also see perlre.