Can that be used for multiple regexes in a file? Like, if you wanted to do s/foo/bar/g and s/y/yah/g on the same file?

Yes. See what happens if you use just perl -pe'undef':

LINE: while (defined($_ = <ARGV>)) {
    undef;
}
continue {
    die "-p destination: $!\n" unless print $_;
}
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So you can just add whatever should be inside the block:

perl -i -pe's/foo/bar/g; s/y/yah/g;' filename
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