my $input = "kw1,kw2; kw3 — kw4‐kw5";

# separator is any string consisting of comma, semicolon,
#  —, – or ‐, bounded by 0 or more whitespace:

my $sep = qr{ \s* (?: , | ; | \&(?:[mn]dash|hyphen); ) \s* }x;

# in the map block, add keyword tags to non-separator items

my @out = map { /$sep/ ? $_ : "<keyword>$_</keyword>" } split /($sep)/, $input;

print join "\n",@out,"";