#!/usr/bin/perl -p

s/(\d+\.\d+\.\d+\.)(\d+)/$1${\($2-1)}/;

##</code><code>##

$ ./decrement.pl infile >outfile

##</code><code>##

s/(\d+\.\d+\.\d+\.)(\d+)/$1.($2-1)/e;

# or (just to show another variant)
s/(\d+\.\d+\.\d+\.)(\d+)/"$1".($2-1)/e;