Solution using Path::Tiny which writes the new file alongside the existing file, then does an atomic rename. See Re^2: Yet another config file editing programme : Tell me how to make it better ! and Re^3: Yet another config file editing programme : Tell me how to make it better !

#!/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 file
+name

my $foundinterface = 0;
path( $dhcpcdfile )->edit_lines( sub
  {
  if( /^\s*profile\s+static_$ip_params{interface}\b.*\n/m ) # alter th
+is section
    {
    $foundinterface = 1;
    }
  elsif( $foundinterface and /^\s*static/m )
    {
    s/^\s*static\s+(\w+)=\K.*/$ip_params{$1}/m or die "failed to chang
+e $1";
    }
  elsif( $foundinterface )
    {
    $foundinterface = 0;
    }
  } );
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