my @items;

foreach my $key (keys %hash){
   push(@items,$key);
}

##</code><code>##

my @items = keys %hash;

##</code><code>##

my $rxMatchItems;
{ local $" = q{|};
   $rxMatchItems = qr{(?:@items)};
}

##</code><code>##

my $rxMatchItems = do { local $" = q{|}; qr{(?:@items)} };

##</code><code>##

$ perl -le'my @items; my $rxMatchItems = do { local $" = q{|}; qr{(?:@items)} }; print $rxMatchItems'
(?-xism:(?:))