Your match doesn't match when readdir returns ., so $1's value isn't changed by the match, and it was previously undefined.

use strict;
use warnings;

my %ext;
opendir my $dh, './' or die $!;
for (grep { -f } readdir $dh) {
   ++$ext{$1} if /\.([^.]+)$/;
}

print "$_ - $ext{$_}\n" for keys %ext;
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Alternate:

use strict;
use warnings;

my %ext;
opendir my $dh, './' or die $!;
++$ext{$1} for map { -f && /\.([^.]+)$/ ? $1 : () } readdir $dh;

print "$_ - $ext{$_}\n" for keys %ext;
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