And if you had 10 arrays you'd want to keep that hash up to date? 20? 30?

The effort is linear with declaring them in the first place: one entry in the hash for each new array.

And if your names are simply consecutive letters, the additional effort is constant: just change the ending character in the slice below.

my @temp=qw(a_1 b_1 a_2 a_3 a_4 e_2 a_5 b_3 a_6 b_4);

my %router;
@router{'a'..'e'} = \(my (@a, @b, @c, @d, @e));
my $keys = join '', keys %router;

foreach my $value (@temp) {
    push @{$router{$1}}, $2 if $value =~ /([$keys])_(.*)/;
}
print "@a <==> @b, @e\n";
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