#!/usr/bin/perl

use strict; https://perlmonks.org/?node_id=11136353
use warnings;
use Path::Tiny;

my $dhcpcdfile = 'fake.353'; # FIXME filename
-w $dhcpcdfile or die "cannot write $dhcpcdfile";

my %ip_params = (
  interface      => "usb0",
  ip_address     => "1.1.1.0",
  routers        => "127.0.0.0",
  domain_servers => "1.1.1.1"
  );

%ip_params = ( %ip_params, # add new data to defaults
  path('inputfile.353')->slurp =~ /^(\w+)=(.*?)\s*$/gm ); # FIXME filename
use Data::Dump 'dd'; dd 'ip_params', \%ip_params;

{ # block scope for locals
local @ARGV = $dhcpcdfile;
local $^I = '.bak'; # make backup, do inplace edit
my $foundinterface = 0;
while( <> )
  {
  if( /^\s*profile\s+static_$ip_params{interface}\b.*\n/m ) # alter this section
    {
    $foundinterface = 1;
    warn "found section for $ip_params{interface} at line $.\n";
    }
  elsif( $foundinterface and /static/ )
    {
    s/^\s*static\s+(\w+)=\K.*/$ip_params{$1}/m or warn "failed to change $1";
    warn "set $1 to $ip_params{$1}\n";
    }
  elsif( $foundinterface )
    {
    $foundinterface = 0;
    warn "ending changes at line $.\n";
    }
  print;
  }
}