If you used the warnings pragma, perl would have told you what is wrong:

C:\>perl -we "print ( split '_' , $myString )[1]"
print (...) interpreted as function at -e line 1.
syntax error at -e line 1, near ")["
Execution of -e aborted due to compilation errors.
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The following works:

C:\>perl -we "$myString = '123_456'; print( ( split '_' , $myString )[
+1] )"
456
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The difference is that print is not a keyword in Perl but a function with optional parentheses, unlike in Python where it is a keyword which doesn't need parentheses.

As an aside, you shouldn't use strings as the first argument to split but regular expressions, as split will interpret the string as a regular expression as well, but passing a regular expression makes it more explicit what happens:

C:\>perl -we "$myString = '123_456'; print( ( split /_/ , $myString )[
+1] )"
456
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