The binding operator - er - binds an operation to be performed to a particular variable instead of $_. I did find this rather confusing at the beginning, because I used to read this as some kind of assignment. See this as if you're assigning "an operation" to the variable.

As for the use of $_[0], Perl passes variables by reference instead of value, which means that you can modify (if possible) the actual variables that are passed to a function. Thus:

sub double_it {
   $_[0] *= 2;
}
my $x = 1;
print "\$x is $x\n";
double_it($x);
print "\$x now is $x\n";
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will multiply $x by two, ending up with the following output:

$x is 1
$x now is 2
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The piece of code you proposed seems some implementation of a templating system; each substitution command replaces occurrences of some placeholder (like %HTTP_HOST%) with the value given by a call to the handleEnvVariable sub with the given parameter (a call to a function is allowed inside the substitution part due to the "e" modifier).