use warnings;
use strict;

package Log; {
    sub new {
        my $self = bless {}, shift;
        my $log_file = shift;
        open my $fh, '>', $log_file or die $!;
        $self->{default_log} = $fh;
        return $self;
    }
    sub log {
        my ($self, $msg, $log) = @_;

        my $log_fh;

        if (defined $log){
            # filename sent in
            open my $fh, '>>', $log or die $!;
            $log_fh = $fh;
        }
        else {
            # use the default fh
            $log_fh = $self->{default_log};
        }
        
        print $log_fh "$msg\n"; 
    }
}

package main; {
    my $log = Log->new('default.log');

    $log->log('default log message');
    $log->log('alternate log file msg', 'not-default.log');
}

##</code><code>##

$ cat default.log
default log message

##</code><code>##

$ cat non-default.log
alternate log file msg

##</code><code>##

    sub log {
        my ($self, $msg, $log) = @_;

        my $log_fh;

        if (defined $log && ref $log ne 'GLOB'){
            # filename sent in
            open my $fh, '>>', $log or die $!;
            $log_fh = $fh;
        }
        elsif (defined $log && ref $log eq 'GLOB'){
            # file handle sent in
            $log_fh = $log;
        }
        else {
            # use the default fh
            $log_fh = $self->{default_log};
        }

##</code><code>##

open my $fh, '>>', 'alternate.log' or die $!;

$log->log('msg', $fh);