It would really help if you showed some code and a specific situation.

"How to get getopt to ignore this field?"
Getopt already ignores what it does not understand. In the option string, you specify that an option takes an argument by the ':',semi-colon character. Getopt takes all of the stuff that it understands out of @ARGV and you process what it left over! In your case, sounds like exactly ONE thing should be left in @ARGV (see below). If you don't specify that an option needs an argument, then Getopt will leave the token after the option alone!

I show some examples that sound like they are close to your situation below. Having an "standalone" argument that looks like an option (-abc or whatever) is a very unusual situation. One way of dealing with this has been posted and there are others. Before rolling further down that rabbit hole, show us an actual example of your problem.

#!/usr/bin/perl -w
use strict;

use Getopt::Std;

my %opts;
my $usage = "usage: options [-a arga] [-b] mandatory_arg";

die "$usage \n" if !getopts("a:b", \%opts);
die "error: arguments=".@ARGV." @ARGV\n$usage \n" if @ARGV !=1;
print "ok\n";

__END__ 
# Wrong, "mandatory" used as -a's argument!
C:\TEMP>option.pl -a  mandatory
error: arguments=0
usage: options [-a arga] [-b] mandatory_arg

# Now ok!
C:\TEMP>option -a arga mandatory
ok

# Error: b doesn't take an arg
# looks like another mandatory arg!
C:\TEMP>option -a -b arga mandatory
error: arguments=2 arga mandatory
usage: options [-a arga] [-b] mandatory_arg

# -b doesn't expect an arg and that causes
# error
C:\TEMP>option.pl -b extra -a arga  mandatory
error: arguments=4 extra -a arga mandatory
usage: options [-a arga] [-b] mandatory_arg

# Now ok!
C:\TEMP>option.pl -b  -a arga  mandatory
ok
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