for, while, do, if, until, unless all operate on BLOCKS of code. If you come from C you will be safe to assume that you always need  { .... } and you always need ; Anyway the general sysntax is

KEYWORD ( CONDITION )
{  # begining of block of code to exec marked by {

   # stuff you want to do goes in here

}  # end of block of code to exec marked by }
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So in short your code is a syntax error(s). Your indentation is also shot. It should look like:

if ( $some_condition ) {
    # some_condition is true
    while( @something && @something_else ) {
        shift @something;
        pop @something_else;
    }
}
else {
    print "Some condition is false!\n";
}
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Code like my second example will compile and is easy to read. Your insistence that nothing has changed means either it never worked, someone rediefined the perl languague while we were not looking or you have changed stuff.

It looks to me as if you are trying to maintain a piece of code written by someone else. I would suggest for a start you google for perltidy and install it, then run it on your code to clean the indentation into something reasonable. That will make your life a lot easier.