# cat test.cfg
newyork 1536000 13 56
california 1536000 67 45
London 456
newyork 1536000 78 87
$ cat spw649958
#!/usr/bin/perl
#
use strict;
use warnings;

use Tie::File;

my $file = q{test.cfg};
tie my @lines, q{Tie::File}, $file
   or die qq{tie: $file: $!\n};

foreach my $line ( @lines )
{
    next unless $line =~ m{^newyork};
    $line =~ s{1536000}{7878787878};
}

untie @lines
   or die qq{untie: $file: $!\n};
$ ./spw649958
$ cat test.cfg
newyork 7878787878 13 56
california 1536000 67 45
London 456
newyork 7878787878 78 87
$