#!/usr/bin/perl

use strict;

my ( $prev, $this ) = ( undef(), undef() );
while( <> ) {
    $this = $_;
    /^>>[^>]/ or mySubst( \$prev );
    print $prev;
    $prev = $this;
}

# finally, process the last line of input which got left over
mySubst( \$prev );
print $prev;

sub mySubst {
    my $sref = shift;
    $$sref =~ s/^>>>/PYTHON_PROMPT/;
}