My favorite (or least favorite) is when Perl tries to be a functional language and treat the name of a function as a first-order value. In normal circumstances, if foo is a function name, and it's followed by a comma, the meaning is to call foo with no args and return the result. For example:

print lc, $/;
##    ^^
## call lc with no args, return the result:
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But in places like map EXPR and grep EXPR, it can mean something very different. This is especially bizarre when foo is a built-in that modifies $_ when it is called with no args:

         map chop, @items;
##           ^^^^
## calls map with first argument = sub{ chop; }
## doesn't change current value of $_

anyotherfunc chop, @items;
##           ^^^^
## calls anyotherfunc with first argument = result of chop($_)
## changes current value of $_ !!
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