my @DELIMS = (qw( _ @ ));
...
... /[@DELIM]/ ...

##</code><code>##

>perl -wMstrict -le
"my @DELIMS = (qw( _ @ ));
 ;;
 my $rx = qr{[@DELIMS]};
 print $rx;
 ;;
 { local $\" = '';  $rx = qr{[@DELIMS]}; }
 print $rx;
 ;;
 my $cc = join '', @DELIMS;
 $rx = qr{[$cc]};
 print $rx;
"
(?^:[_ @])
(?^:[_@])
(?^:[_@])