d:\>perl -MO=Deparse -e "do {s/\s+/\t/; s/elementary/my dear Watson/g;
+} for @array;"
foreach $_ (@array) {
    s/\s+/\t/;
    s/elementary/my dear Watson/g;;
}
-e syntax OK
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You machine probably was overloaded with some extra tasks second time. And I beleive multiline version compiled into the same code internally.