but a little reading (on google) about using constants seems to indicate that is very little speed improvement.

It very much depends upon what you are doing with them.

For example:

use constant BOOL => 0; 
our $BOOL = 0; 
cmpthese -1,{ 
    const => q[ 
        for(1..1e6){ 
            if(BOOL){ 
                my $n = 1; 
                $n *=$_ for 1 .. 1000;
            } 
        } 
    ], 
    var => q[ 
        for(1..1e6){ 
            if($BOOL){ 
                my $n = 1; 
                $n *=$_ for 1 .. 1000;
            } 
        } 
    ] 
};;
        Rate   var const
var   15.1/s    --  -22%
const 19.4/s   29%    --
[download]

Note: The code inside the if block isn't executed in either case; but in the variable version, the value of $BOOL has to be tested every time around the loop in case it changes.

But with the constant, Perl can see that the value cannot change, so it optimises away the entire if statement; so that 29% performance gain comes simply from not having to test a value that will never change 1e6 times.

If you want to know what difference your use will make don't ask for opinions; benchmark it.

Some you'll gain, some you won't; but you'll never loose.