use strict;                                         # use this always. no excuses.
use warnings;                                       # same with this.

my @params; my @charam; my @actual;                 # declare all your variables!!
my $i; my $j;                                       # declare all your variables!!

open FILE, "params.txt" or die "unable to open\n"; 
for($i=1,$j=1; <FILE>; $i++, $j++) { 
	chomp; 
	(@params[$i], @charam[$j]) = split(':');    # use $i and $j here, not i and j!!!
	$actual[$i]=$charam[$j];                    # better use $actual[$i] instead of @actual[$i] (same on line above!)
	print "inside $actual[$i]\n"; 
} 
print "out side @actual\n";                         # it works, but gives an error as $actual[0] is undefined
close(FILE);