This is similar to Anonymous Monk's suggestion but builds a lookup table. Tables can often simplify your logic and avoid a string of if blocks.

#! /usr/bin/perl

use strict;
use warnings;

use Data::Dumper;
$Data::Dumper::Indent = 2;

my @ranges = (
#  low   high  conf_file
  [ 204,  381,  2],
  [  22,  199,  4],
  [ 513,  690,  6],
  [ 823, 1000,  8],
  [1067, 1244, 10],
  [1311, 1488, 12],
  [1555, 1732, 14],
  [1799, 1976, 16],
  [2043, 2220, 18],
  [2287, 2464, 20],
);

my %lookup;
for my $range (@ranges){
  my ($low, $high, $slot) = @{$range};
  for my $index ($low..$high){
    $lookup{$index} = $slot;
  }
}

#print Dumper \%lookup;
#exit;

my @ifindex = (204, 22, 513);
my ($msan_slot, $msan_port) = translate_ifindex(@ifindex);

sub translate_ifindex {
  
  my @ifindex = @_;
  
  my $ports_ini_dir = "/home/portal/bin/conf/snmp_set";
  my (@msan_slot, @msan_port);
  
  for my $t (0..$#ifindex) {
    if (exists $lookup{$t}){
      my $cnf_file = sprintf(q{%s/snmp_ports_slot%02d.txt}, $ports_ini
+_dir, $lookup{$t});
      my $cfg = Config::IniFiles->new( -file => $cnf_file)
        or die q{cant open *$cnf_file*};
      my $slot = $cfg->val($ifindex[$t], "slot");
      my $port = $cfg->val($ifindex[$t], "port");
      push @msan_slot, $slot;
      push @msan_port, $port;
    }
  }
  
  return \@msan_slot, \@msan_port;
  
}
[download]

Untested due to not having the config files.

updated: corrected table syntax and sprintf format string