I think it can be simplified (subject to your <256 constraint) using printf:

use warnings;
use strict;

print "Enter a decimal number, less than 256, to be converted to a bin
+ary number: ";

chomp(my $dec = <STDIN>);

printf "%08b\n", $dec;
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See also the FAQ:

perldoc -q convert
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But, yours is an interesting use of the <=> operator.