use strict;
use warnings;

my $outfile = "test.txt";
open (my $ofh, ">$outfile") or die;

foo ($ofh); #Prints to the file pointed by $ofh

foo(); #Prints to STDOUT

sub foo
{
    my $filehandle= shift;
    $filehandle ||= \*STDOUT ;
    select($filehandle);

    print  <<"EOF";
This is a test, it is only a test. 
In the event of an emergency, run around in a panic.
EOF

}