Your pattern matches 0 and 100 even though neither are between 0 and 100. Is your question wrong or is your solution wrong?

Solutions for "integers between 0 and 100":

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

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

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

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

Solutions for "integers from 0 to 100":

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

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

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

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

In all cases, you'll need to anchor the pattern as appropriate.

The first solution of each set accepts leading zeroes. Easily changed.
The bottom three solutions of each set don't accept leading zeroes. Easily changed.

Update: Oops. Accidentally omitted a "?" from my patterns.