How do I expand function calls in a string?

This is documented in the perlref manpage. To interpolate a subroutine call (in a list context) into a string: 

    print "My sub returned @{[mysub(1,2,3)]} that time.\n";

If you prefer scalar context, similar chicanery is also useful for arbitrary expressions: 

    print "That yields ${\($n + 5)} widgets\n";