while(<FILE>) #open a file
{
        if ( /$start/ .. /$file/ ) { # if line match [start] till
[files]
                do something with $_
        }
        if (/$file/ .. /$end/  )
        {
           do something with $_
        }

[start]
...
[files]
...
[end]

<code>
import sys
import re

start, files, end = map(re.escape, ["[start]", "[files]", "[end]"])

class Section(object):
    def __init__(self, start, end):
        self.start = re.compile(start).match
        self.end = re.compile(end).match
        self.inside = False
    def __contains__(self, line):
        result = self.inside
        if result:
            if self.end(line):
                self.inside = False
        else:
            if self.start(line):
                result = self.inside = True
        return result

first = Section(start, files)
second = Section(files, end)
for line in sys.stdin:
    line = line[:-1]
    if line in first:
        # your code
    if line in second:
        # your code
</code>

However, the simpler

<code>
#untested
import sys

start, files, end = "[start]", "[files]", "[end]"
keys = set([start, files, end])

key = None
for line in sys.stdin:
    line = line[:-1]
    if line in keys:
        key = line
    elif key == start:
        # your code
    elif key == files:
        # your code
</code>

might work even better because 'your code' doesn't get to see the sections'
begin/end markers.

You could create a flip-flop with a python closure (t_cond and f_cond
are functions that take a value and return True of False)