How about dupping the filehandle:

sub open_logfile {
  my $filename = shift;
  open STDOUTBACK, '>&STDOUT' or die "Can't dup STDOUT\n";
  open STDOUT, '> ' . $filename or die "can't write to $filename: $!\n
+";
}
[download]

In this case any attempt to print "foo\n" or print STDOUT "foo\n" will go to the logfile.

Or just use select:

sub open_logfile {
  my $filename = shift;
  open FH, '> ' . $filename or die "Can't write to $filename: $!\n";
  select FH;
}
[download]

In the second case print "foo\n" will print where you want but print STDOUT "foo\n" will still go to the real STDOUT.