As an alternative to the regular expression ikegami showed you, you can use rindex and substr to locate and extract your text. You might find this a little easier to understand (after you read the manual pages). For example:

use strict;
use warnings;

my $pos = 21;
my $str = "ABCDEFANABCDEFGHIJKLMNOPQRSTVVWXWZ";

my $pos_fan = rindex($str, "FAN", $pos);
print substr($str, $pos_fan + 3, 3) . "\n";
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produces

ABC
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