***This is not an RE***. You need m"this" syntax to make it an RE. The only time you can not use m is if you have syntax $foo =~ /this/ Using the / char as the RE delimiter makes the m implicit. Your 'RE' uses " as the delimiter so needs the m. It does not do what I expect the original programmer thought it would. Here is what it does:

use warnings;
my %flows = ( '0' => 1, '00' => 2, '0$' => 3, '0$more' => 4, 'foo0$' =
+> 5 );

foreach (keys %flows) {
    # loop through keys of %flows setting $_ to each key in turn
    if (!($_ =~ "^0\$")) {
        print "Orig $_ not matched\n";
    } else {
        print "Orig $_ matched\n";
    }

    if (!($_ =~ m"^0\$")) {
        print "RE   $_ not matched\n";
    } else {
        print "RE   $_ matched\n";
    }
    print $/;
}


__DATA__
Orig 0 matched
RE   0 not matched

Orig 00 not matched
RE   00 not matched

Orig 0$more not matched
RE   0$more matched

Orig 0$ not matched
RE   0$ matched

Orig foo0$ not matched
RE   foo0$ not matched
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I am intrigued it is not a syntax error or warning. It appears that the intent is to do something if the hash key does not start with the 2 literal chars '0$'. It does not do that as you can see.

You need m"this" syntax to make it an RE

Actually, not.

print "Found '$1'" if 'one two three' =~ 'one(.*)three';
Found ' two '
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---

Examine what is said, not who speaks.

"Efficiency is intelligent laziness." -David Dunham
"Think for yourself!" - Abigail

While your example works as shown it is contrary to the docs (perlop) and also fails to explain the observed behaviour noted above.....

If ``/'' is the delimiter then the initial m is optional. With the m y
+ou can use any pair of non-alphanumeric, non-whitespace characters as
+ delimiters.

C:\>type test.pl
$re =~ "^0\$";
$re =~ m"^0\$";
$re =~ '^0\$';
$re =~ m'^0\$';
$re =~ <^0\$>;
$re =~ m<^0\$>;

C:\>perl -MO=Deparse test.pl
$re =~ /^0$/;
$re =~ /^0\$/;
$re =~ /^0\$/;
$re =~ /^0\$/;
$re =~ /CORE::GLOBAL::glob('^0$', 0)/;
$re =~ /^0\$/;
test.pl syntax OK
C:\>
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The quote and other chars can also be used without m but the results may not be intuitive.....

cheers

tachyon

To BrowserUk ,I don't understand how to get interpolated as string when it is "^0\$".Can you explain more detail? thanks!

my test script like this:

$_ = '0$';
if($_ =~ "^0\$")
{
 print "find\n";
}
else
{
 print "not find\n";
} 

result is:
not find
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