sub fill_template {
    my( $template, %values ) = @_;
    my $re_values = join "|", map { "\b$_\b" } reverse sort keys %values;
    $template =~ s!($re_values)!exists $values{ $1 } ? $values{ $1 } : $1!gre;
}

for my $linenumber (0..$#arr) {
    my $user_data = fill_template( $data, USER_C => $arr[$i], IP => $arr1[$i] );
    print $user_data; # well, output to a file, but that's another problem
}