You want to make changes, so you want the substitution operator (s///). And you want the replacement to be the result of a Perl expression, so you need to use the "e" modifier.

use Time::Local qw( timelocal );

s/(\d{4})-(\d{2})-(\d{2})/ timelocal(0,0,0, $3,$2-1,$1) /ge;
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but I continue to get syntax errors (I assume because you can't call subroutines inside a regex).

Maybe there's something wrong with the bits you omitted (like an unescaped "/"?) since the bits you did post are fine. Next time, please post the actual code and error messages.

Well, except that you're using "&" on your function call. Why are you telling Perl to ignore prototypes?