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It looks like you might be confusing variable names and strings. "$pax_$plus" simply creates a string. If $pax is "foo", and $plus is "100", then "$pax_$plus" creates the string "foo_100" "100" , not the variable $foo_100. (see moritz's reply below for why the resulting string is "100" and not "foo_100")

When you want to retrieve data associated with a series of consecutive numbers (i.e. 1 to 4), you need to use an array, like this:

# use these lines at the start to get Perl help you find
# bugs
use strict;
use warnings;

# Note: declare your variables with "my"
# This helps Perl catch any misspellings in variable names
# and reminds you to set initial values for everything

# price for 0, 1, 2, 3, 4 persons
my @pax = (0, 100, 260, 300, 450);
my $plus = 1;
my $currencycode = 'ZAR';
my $sleeps='';
my $maxadults = 4;

while ($maxadults >= $plus) {

  my $paxing = $pax[$plus];  #get $plus member of @pax array

  $sleeps = $sleeps . "$currencycode$paxing for $plus persons<br>";

  $plus++;
}
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