...
 my ($fnNew, $fnOld) = ('test.cfg', 'test.cfg.bak');
 rename $fnNew, $fnOld or die "can't rename $fnNew, $!";

 open my $fin,  '<', $fnOld || die "$fnOld - $!";
 open my $fout, '>', $fnNew || die "$fnNew - $!";
 
 while( my $line = <$fin> ) {
    $line =~ s/(newyork\s+)1536000/${1}7878787878/;
    print $fout $line
 }
 close $fin; close $fout;
 ...