assuming you have your file in an array  @test(for example purposes) you could do someting pretty simple:

my $end = @test;
my $cnt = 0;
until ($cnt == $end)
    
    if (($test[$cnt] =~ /^>>>\s/) && ($test[$cnt+1] !~ /^>>\s/)){
        $tst[$cnt] =~ s/^>>>/PYTHON_PROMPT/;
    }
    $cnt++;
}
[download]