If you want to replace the rightmost 2 digits, one way is by using substr:

my $extracted_digits = substr( $var, -2, 2, $newdigits );
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For your specific example:

use strict;
use warnings;

my $var = 200504250955;
my $newdigits = 67;
print "$var\n"; # 200504250955
my $extracted = substr( $var, -2, 2, $newdigits );
print "$var\n"; # 200504250967
print "$extracted\n"; # 55
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