What have you tried? How did it fail? What specific detail are you having trouble with? See I know what I mean. Why don't you?. As a starting point you may find it helps to refactor your expression as:

use strict;
use warnings;

my @opts = qw(K1 ASC GEB N1);
my $alts = join '|', @opts;
my $re = qr{^(?:$alts)_(\w*\d*)$};

print "Matched $1\n" if 'GEB_S1' =~ $re;
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