It would have been useful if you'd posted the original code. Here it is:

use Modern::Perl;

say +(split ':')[0] while (<DATA>);
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say (and the same goes for print which has fuller documentation) can take a FILEHANDLE as its first argument.

say FILEHANDLE LIST
print FILEHANDLE LIST
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In short, the '+' indicates that the FILEHANDLE (typically STDOUT) is to be assumed and the arguments following the '+' are the LIST to be output.

Edit: Sorry, that last paragraph was poorly worded. Hopefully, this next one is better. Also see the example code in the Update at the end.

If the FILEHANDLE is omitted, the '+' indicates that the arguments following the '+' (once expanded) are the LIST to be output. If the FILEHANDLE is included, the '+' is not required.

More complete details can be found in print and perlop - Symbolic Unary Operators.

Regarding Modern::Perl, that will effectively do the use feature 'say'; for you; if you comment that out, you will need to do it yourself. I think that's what you were getting at; although, I did get a little lost in the multiple negatives and conditions: "the code works if ... but it doesn't unless ...".

Update: I felt my response might have been misconstrued as being contrary to the previous responses. This is certainly not my intention; in fact, there weren't any responses when I wrote mine. The links to documention that I provided do give more detailed information; however, the following code examples may clarify what I was saying.

$ perl -Mstrict -Mwarnings -E 'my @x = (1,2,3); say (@x)[1]'
say (...) 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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$ perl -Mstrict -Mwarnings -E 'my @x = (1,2,3); say STDOUT (@x)[1]'
2
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$ perl -Mstrict -Mwarnings -E 'my @x = (1,2,3); say +(@x)[1]'
2
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