If the compiler can determine at compile time that an expression will never be evaluated, it will remove the code. For example:

use strict;
use warnings;

print "Hello world" if 0;
exit 1 if 0;
exit 0;
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looks like this to the Perl compiler:

$ perl -MO=Deparse /tmp/compile.pl

use warnings;
use strict 'refs';
'???';
'???';
exit 0;
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The two .. if 0; lines are turned into void strings which take basically no time to evaluate.