http://www.perlmonks.org?node_id=662677


in reply to Re^3: unless versus if ( ! ) inside a subroutine
in thread unless versus if ( ! ) inside a subroutine

Conditionals are not always evaluated. The compiler can sometimes decide to phrase things differently.
U:\> perl -MO=Deparse -le "$x = 5; if ($x) { print $x }" $x = 5; if ($x) { print $x; }
U:\>perl -MO=Deparse -le "$x = 5; if (0) { print $x }" $x = 5; '???';
U:\>perl -MO=Deparse -le "$x = 5; if (1) { print $x }" $x = 5; print $x;;
U:\>perl -MO=Deparse -le "$x = 5; if (! 0) { print $x }" $x = 5; print $x;;

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Re^5: unless versus if ( ! ) inside a subroutine
by almut (Canon) on Jan 16, 2008 at 15:11 UTC

    Sure, but in those cases, constants are being tested in the conditional, which allows the compiler to make optimisations. In the OP's example, though, $v is a regular (mutable) scalar variable, which could change its value at runtime (after the compiler has done its work)...