my @items;

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

my @items = keys %hash;
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my $rxMatchItems;
{ local $" = q{|};
   $rxMatchItems = qr{(?:@items)};
}
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Or simply:

my $rxMatchItems = do { local $" = q{|}; qr{(?:@items)} };
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Because your %hash is empty your pattern match becomes:

$ perl -le'my @items; my $rxMatchItems = do { local $" = q{|}; qr{(?:@
+items)} }; print $rxMatchItems'
(?-xism:(?:))
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And the pattern (?-xism:(?:)) will match everything.

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

Oh noes :)

my $rxMatchItems = join '|', map quotemeta, @items;
$rxMatchItems = qr/$rxMatchItems/;
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I'm guessing from the variable name and the use of quoting constructs that the OP grabbed that bit of code from one of my solutions, probably one where @items contained values known not to need quotemeta'ing. Invariable application of quotemeta without any consideration of whether it is necessary is just another form of cargo cult programming. I don't think we can tell from the OP's code whether it is required or not. Even if it is required, the do block construct is as valid as using join.

my $rxMatchItems = do
   {
       local $" = q{|};
       qr{(?:@{ [ map quotemeta, @items ] })};
   };
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Cheers,

JohnGG

use warnings;
use strict;
use Data::Dumper;
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Please don't read the previous posting as "using strict and warnings is nonsense", the opposite is true: Always use strict and warnings (except in rare situations like Perl golf). The use of Data::Dumper is nonsense here, and it really looks like cargo cult.

Alexander

--
Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)

You always add in use Data::Dumper; whenever you want to debugprint a datastructure and remove it as soon as you remove all debug prints that need it? Really? I quite often leave it there knowing that sooner or later I'll need it again.

Sometimes the holly war against cargo culting is a bit cargo cultish.

Jenda
Enoch was right!
Enjoy the last years of Rome.

The hash is populated from a DB.

I would think that the first thing is to decide whether you even need to write any kind of program or not (Perl or otherwise)! I figure you are on a Unix type machine. There is a standard program that does what you want called "grep".

Type "man grep", "man egrep" at the command line to get some hints. "grep P40303 *.datafile" will output all lines containing P40303 in all files ending in ".datafile".

But if you must, here is some Perl code...

#!/usr/bin/perl -w
use strict;

my @items = qw (P40303 Q99436 X1234 W9765543);
my $regex = join ("|",@items);
print $regex; # to see what this does 
              # put something like this in your "grep"
              # P40303|Q99436|X1234|W976554
while (<>)
{
   print if m/$regex/;
}

__END__
Perl 5.10 is pretty smart. 
I think that the /o option is not necessary here.
I don't think more complex syntax's are either.
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Probably because your regex is nonsense

Harsh, but helped