my $pat = qr/
    ([0-9]+)
    (?(?{ my $n = $^N; $n <= 0 || $n >= 100 })(?!))
/x;

##</code><code>##

my $pat = qr/0|[1-9][0-9]?/;

##</code><code>##

my ($pat) =
    map qr/$_/,
    join '|',
    #map quotemeta,
    1..99;

##</code><code>##

use Regexp::List qw( );
my $pat = Regexp::List->new()->list2re( 1..99 );

##</code><code>##

my $pat = qr/
    ([0-9]+)
    (?(?{ my $n = $^N; $n < 0 || $n > 100 })(?!))
/x;

##</code><code>##

my $pat = qr/0|100|[1-9][0-9]?/;

##</code><code>##

my ($pat) =
    map qr/$_/,
    join '|',
    #map quotemeta,
    0..100;

##</code><code>##

use Regexp::List qw( );
my $pat = Regexp::List->new()->list2re( 0..100 );