$i = 2;
$j = 4;
print "$_\n" for ($i..$j);
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Output:

2
3
4
[download]

Update: I missed what was obvious to choroba. Not so much a flip-flop as a filter:

# show lines 2 thru 4
while (<>) {
  print if 2..4
}

# this will print every line
$i = 2; 
$j = 4;
while (<>) {
  print if $i..$j
}
[download]