The usage for <=> here in the masking expreasion ($decimal & 0x80) is to test if this 8th bit is zero or one, also you can do it like:

#!/usr/bin/perl -w 
use strict; 
use warnings; 

print "Enter decimal number less than 256:"; 
my $decimal; 
$decimal=<STDIN>; 
chomp $decimal; 
# $decimal += 0;
print (($decimal & 0x80)? (1) : (0) );    # to test the 8th bit
print (($decimal & 0x40)? (1) : (0) );    # to test the 7th bit
print (($decimal & 0x20)? (1) : (0) );    # to test the 6th bit
print (($decimal & 0x10)? (1) : (0) );    # to test the 5th bit
print (($decimal & 0x08)? (1) : (0) );    # to test the 4th bit
print (($decimal & 0x04)? (1) : (0) );    # to test the 3th bit
print (($decimal & 0x02)? (1) : (0) );    # to test the 2nd bit
print (($decimal & 0x01)? (1) : (0) );    # to test the 1st bit
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Are so many parentheses needed with the lower precedence  ? : Conditional Operator? Also, rather than using what looks like an unrolled loop for printing bits, is there any way to use a while- or for-loop here?

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Give a man a fish:  <%-(-(-(-<

Yes, you can drop all the parens when you switch to the ternary operator. And yes, it can be done as a loop, which also removes the 8-bit restriction:

$ cat 1129169.pl
#!/usr/bin/env perl
use 5.010; use strict; use warnings; 

sub dec_to_bin {
    my $dec = shift;
    print "$dec : ";
    unless( $dec =~ /^\d+$/ ){
        say "Not a whole number!";
        return;
    }
    my $x = 1; my $bin = '';
    while(1){
        my $bit = $dec & $x;
        $bin = ($bit ? 1 : 0) . $bin;
        $dec -= $bit;
        last unless $dec;
        $x <<= 1;
    }
    say $bin;
}
dec_to_bin($_) for @ARGV;

$ perl 1129169.pl 0 1 4 255 65536 abc
0 : 0
1 : 1
4 : 100
255 : 11111111
65536 : 10000000000000000
abc : Not a decimal number!
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Aaron B.
Available for small or large Perl jobs and *nix system administration; see my home node.