my $pseudo_file = "line1\nline2\n";
open my $rfd, '<', \$pseudo_file or die "open1: $!";
open my $wfd, '>', 'foo' or die "open2: $!";
print $wfd $_ while <$rfd>;      ## or 'print $wfd (<$rfd>);' if file isn't large
close $wfd;
close $rfd;