$ cat test.conf 
fieldX:fieldY=123
fieldA:fieldB=234

$ cat t.pl
#!/usr/bin/perl

use warnings;
use strict;

{
    package MyConfig;
    
    use Tie::File;
    
    sub new {
        my ($class, $file) = @_;
    
        tie(my @config, "Tie::File", $file)
          or die("Failed to read ", $file, "\n  ", $!);
    
        return bless \@config, $class;
    }
    
    sub setting {
        my $self = shift;
        my $line = shift;
    
        if (3 == grep { defined($_) } @_)
        {
            $self->[$line] = "$_[0]:$_[1]=$_[2]";
        }
        else
        {
            return split /[:=]/, $self->[$line];
        }
    }
    
    sub DESTROY { untie @{ $_[0] } }
}

use Data::Dumper;

my $s = MyConfig->new("test.conf");

my @set1 = $s->setting(0); # at line 1
my @set2 = $s->setting(1); # at line 2

print Dumper \@set1, \@set2;

$s->setting(1, "foo", "bar", "777"); # modify line 2
$s->setting(4, "baz", "red", "999"); # add line 5

__END__


$ t.pl
$VAR1 = [
          'fieldX',
          'fieldY',
          '123'
        ];
$VAR2 = [
          'fieldA',
          'fieldB',
          '234'
        ];

$ cat test.conf
fieldX:fieldY=123
foo:bar=777


baz:red=999