Regex match information has a somewhat complex interaction with lexical scope. The 'grep' case fails because the scope of the expression is left and re-entered. The 'for' case succeeds because the scope of the body of the 'for' is never effectively left/entered during the running of the loop (where what "effectively" means is intentionally vague).

My evidence to support this claim is that adding another block of scope to the 'for' loop breaks it as well:

use strict;
my $if = 1;
my @list= ("a".."c","DBIC","A".."C","DANCER","a".."c");
my @result;
for (@list) {
    if(  $if  ) {   # Comment this out
        push @result,$_ if /DBIC/.. /DANCER/ and ! //;
    }               # Comment this out
}
print $_, $/ for @result;
__END__
[download]

Comment out the two indicated lines to again see A .. C output. As is, the above code outputs nothing (similar to the 'grep' case).