Good catch (++). As dave_the_m suggested we can use brackets in this case:

#!/usr/bin/env perl 
use strict;
use warnings;
use utf8;

my @cases = qw/ABA SCO ACC PHC GHF/;
my $re = join '|', @cases;

for my $text ('SCO', 'ENDOSCOPE', 'Microsoft') {
    print "$text found (grep)\n" if grep {$text eq $_} @cases;
    print "$text found (regex)\n" if $text =~ /^($re)$/;
}
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