for my $a (-2..10, 0.2, -0.2, 1 + 0.25e-15) {  
        $_ = unpack("b*", pack("d", $a));   # double to bit-pattern 
        s/(.{8})/$1 /g; tr[0][.];           # grouping&fmt.
        $_ = reverse $_;                    # formatting
        s/^(..)(.{12})(.*)/Sgn:$1  Exp: $2  Mant:(1,)$3/; 
        print "$a\t -- $_\n";
}

##</code><code>##

my $a = 1+0.25e-15;# hidden bit and last bit of mantissa: 1
print "Test                : is int.?   correct?\n";
print "a != int(a)         : no         yes\n" if $a != int($a);
$d = abs($a - int($a));
print "abs(a-int(a) < 1e-15: yes        no!\n" if $d<1e-15;
print "abs(a-int(a) < 1e-40: yes        ???\n" if $d<1e-40;
print "abs(a-int(a) > 0    : no         yes\n" if $d>0;
printf("epsilon < %g required (here!)\n", $d);
__END__
Test                : is int.?   correct?
a != int(a)         : no         yes
abs(a-int(a) < 1e-15: yes        no!
abs(a-int(a) > 0    : no         yes
epsilon < 2.22045e-16 required (here!)