use Tie::File;
tie my @lines, 'Tie::File', 'filename' or die $!;

for (my $i = 1; $i <= $#lines; $i++) {
    if ($lines[$i] =~ /^\+/) {
        $lines[$i - 1] =~ s/\n/substr $lines[$i], 1/e;
        splice @lines, $i, 1;
        redo;
    }
}