I recommend looking at perlre on perldoc.perl.org first. There are two main operators m/ and s/ for match and substitute. It's a lot to learn, but no time like the present. To take an example in detail: I want to change all digits into X in the string $s:

$s =~ s/\d/X/g;
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The =~ announces a regex operator, in this case s for substitute. '/' are the most common delimiters. You need two for match and three for substitutions. \d is the digit token, X is literal and the g at the end is for match all occurrences.

There are lots of tokens and modifiers. In principle a complex matching is achieved simply by concatenating terms together e.g. ^\d+\S requires the \d+ to start at the beginning and the \S would be a non-space after the digits -- so not a digit which would have been consumed by the \d+ term.

Bon voyage on your journey through perlre!