This is what I would do:

my @devices;

open (FILE,"config.txt") or die "cannot open file : $!";

# Always put a label on your loop when using next
CONFIG_LOOP:
while ( my $line = <FILE> ) {
  
  # Never use $_ directly
  if ( $line =~ /^(\w+)\s+/ ) {
    
    # Adding to the array
    push( @devices, "$1 => " );
  }
  elsif ( $line =~ (/inet\saddr:(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1
+,3})/) ) {
    
    # Adding the IP address to the last known device
    $devices[-1] .= "$1.$2.$3.$4\n";
  }
  else {
    
    # Skipping other lines
    next CONFIG_LOOP;
  }
}

close (FILE);

foreach my $device ( @devices ) {
  
  print $device;
}
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