You're likely looking at a script not written in or for Perl. If you use warnings, Perl will tell you what it sees instead:

perl -we "System.gc"
Useless use of a constant in void context at -e line 1.
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What Perl sees here is the concatenation of two bareword strings:

perl -we "print System.gc"
Unquoted string "gc" may clash with future reserved word at -e line 1.
Systemgc
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Perl has no garbage collector, as it uses reference counting only.