use strict;
use warnings;

my $string = 'char(123)+char(107)+char(112)+char(43)';

# change all 'char' to 'chr' (the Perl function)
$string =~ s/char/chr/g;

# split the string on '+', eval each part, then join the resulting
# characters back into a single string
$string = join( '', map { eval $_ } split( /\+/, $string ) );

print "$string"; # {kp+