Mostly springing from tye's solution, but it avoids the memory growth.

$\ = "\n";
for $n ( 4 .. 6 ){ 
    print for '0' x $n .. '9' x $n;
}
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And if you needed different 'digits' then

$\ = "\n";
my @digits = 'a' .. 'j';

for my $n ( 4 .. 6 ) {
    print @digits[ split '', $_ ] 
        for '0' x $n .. '9' x $n;
}
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