AM,

Not very portable but some fun with Inline on I386 Linux:

#!/usr/local/bin/perl

use Inline C;

print dec2bin(13) . "\n";
print idec2bin(8589934593) . "\n";

sub dec2bin {
        return ( sprintf "%b", shift );
}

__END__
__C__
void idec2bin ( SV *num ) {
        long long count;
        long long MASK;
        long long var;
        char *beg;
        char *end;

        count = sizeof( var ) * 8;
        MASK  = 1LL << (count-1);
        beg   = SvPV( num, PL_na );
        var   = strtoll( beg, &end, 10 );

        while( count-- ) {
                printf( "%d", (var & MASK) ? 1 : 0 );
                var <<= 1;
        }
}
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-derby