foreach ( @array ) {
    if ( exists $hash{$_} ) {
        # do something
    } else {
        # do something else
    }
}

##</code><code>##

my %hash = ( ... );
while ( $_ = get_item() ) {
    if ( exists $hash{ $_ } ) { ### update: fixed syntax error typo
        # do something
    } else {
        # do something else
    }
}