m#.*,# is the same as /.*,/ but the slashes were already used by s///, and the vars are $\ (set by perl -l to "\n", see perlrun) and $&. Same thing:

perl -ple 's{ [;\s]+ }{ m{.*,}; $/ . $& }egx'
perl -ple '($a)=/(.*,)/; s/[;\s]+/\n$a/g'
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