Perhaps you want:

perl -e 'if("42"=~ /^[[:digit:]]+$/) {print "42 is a digit\n"} else {p
+rint "42 is not a digit\n"};'
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the inner [] is part of the charclass name. And do not mix ' and " for oneliners. Just start with ' and use " or q or qq in your oneliner on UNIX.