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if( not defined $ENV{SERVER_ENVIRONMENT} or not $ENV{SERVER_ENVIRONMEN
+T} ) {
    print $fh_log "Environment variable \"SERVER_ENVIRONMENT\" is requ
+ired to be set up. \n";
    die "Environment variable \"SERVER_ENVIRONMENT\" is required to be
+ set up. \n";
}

use strict;
use warnings;
...
foreach (sort keys %ENV) {
    print $fh_log "$_ $ENV{$_}\n";
}