my $var = '' if 0;

##</code><code>##

if (0) { my $var = ''; }

##</code><code>##

bash$ perl -e 'for(1..3){my $i if 0; print $i++, "\n"}'
0
1
2

bash$ perl -e 'for(1..3){my $i if 1; print $i++, "\n"}'
0
0
0