use strict; 
use warnings;
use List::Util qw[ reduce ];

my @pfs = ( 3,5,7,19 );
my $num = reduce { $a * $b } @pfs;
my $root = sqrt $num;
print "N=$num, R=$root\n";

#the rest is the same as you had it

##</code><code>##

use strict; 
use warnings;
use List::Util qw[ reduce ];

my @pfs = ( 3,5,7,19 );
my $num = reduce { $a * $b } @pfs;
my $root = sqrt $num;
print "N=$num, R=$root\n";

my $near = reduce{ 
    abs($root - $a * $b) < abs($root - $a) ? $a*$b : $a
} reverse @pfs;

print abs($root - $near) > abs($root - ($num/$near))
    ? $num/$near
    : $near
;