my %main_file=(
                  USER <= \@user_file,
                  TEST <= \@test_file,
                  TRIM <= \@trim_file,
                  );

##</code><code>##

push(@{$main_file{"USER"}[0]}, $string);

##</code><code>##

> perl -Mstrict -MData::Dumper -wE '
    my %main_file = ( USER => [ [], [], [] ] );
    push @{$main_file{"USER"}[0]}, "string ;)";
    print Dumper \%main_file;
    '
$VAR1 = {
          'USER' => [
                      [
                        'string ;)'
                      ],
                      [],
                      []
                    ]
        };
>