use strict;
use warnings;

my $string='
hi hello @begintobe
replace @end asas x x x x x zxzxax
hi hello hi hellooo
@beginthis has tobe
replaced @end ...';

my @replace= ("replacestring1","replacestring2");

$string =~ s|\@begin.*?\@end
            |shift @replace // '[null]'
            |sexg;

print $string;