select $| Output with say

select the filehandle not use the default STDOUT;

open FILE, ">log.txt" or die "cannot open $!";
select FILE;
$| = 1;
print "this is to log";
select STDOUT;
print "this is to stdout\n";

after change the filehandle, better to change back.
$| special variable is flush the buffer.

Perl 5.10 borrows the say built-in from the ongoing development of Perl 6 (which may
have borrowed its say from Pascal’s println). It’s the same as print, although it adds
a newline to the end. These forms all output the same thing:
use 5.010;
print "Hello!\n";
print "Hello!", "\n";
say "Hello!";
This is especially handy in the common case of simply wanting to put a newline after whatever you want to output:

To interpolate an array, you still need to quote it though. It’s the quoting that puts the
spaces between the elements:
use 5.010;
my @array = qw( a b c d );
say @array;   # "abcd\n"
say "@array"; # "a b c d\n";
Just like with print, you can specify a filehandle with say:
use 5.010;
say BEDROCK "Hello!";