The following snippet is almost certainly not what you want, but I find it instructive...

#!/usr/bin/perl

use strict;

my $numrgx = qr/^-?[\d.]+(?:e[+-]\d{2})?$/;

my @try_these = ( 
    "0.2000000000000001",
    "0.3000000000000001"
);

for ( @try_these ) {
    my $x = my $y = $_;
    $y .= "1";

    my $same = ( $x eq $y );
    if ( ! $same and
         ( $x =~ /^-?\.?\d/ and $x =~ /$numrgx/ ) and
         ( $y =~ /^-?\.?\d/ and $y =~ /$numrgx/ )) {
        $same = ( $x == $y );
    }
    my $cmp = ( $same ) ? "is the same as" : "differs from";

    print "$x $cmp $y\n";
}
[download]

For me (macosx 10.4 on intel core 2 duo, perl 5.8.6), the output is:

0.2000000000000001 differs from 0.20000000000000011
0.3000000000000001 is the same as 0.30000000000000011
[download]

Have fun...

BTW, the reason for the funny looking condition -- checking matches for both  /^-?\.?\d/ and for the more elaborate regex -- was that it seemed like an easy way to keep $numrgx relatively clear and simple, while still making sure that the string contains at least one digit (which $numrgx by itself does not guarantee).

(updated the initial match condition that checks for at least one digit, so that it would only accept "\d", "-\d", ".\d" or "-.\d" at the beginning of the string; of course, $numrgx is still faulty, since it allows multiple periods)

Another update -- I knew the regex approach above was silly, and I was curious to have a version that would make it easier to test for more boundary cases (where floating point limitations blur the "same/different" distinction), so here's a version with a better regex -- still not perfect, I suspect, but more fun because of the @ARGV options:

#!/usr/bin/perl

use strict;

my $numrgx = qr/^-?(?:\d+\.?\d*|\d*\.\d+)(?:e[+-]?\d{1,2})?$/;

my @try_these = ( @ARGV == 2 ) ? ( $ARGV[0] ) :
( 
    "0.2000000000000001",
    "0.3000000000000001"
);

for ( @try_these ) {
    my $x = $_;
    my $y = ( @ARGV == 2 ) ? $ARGV[1] : $_ . "1";
    my $same = ( $x eq $y );
    if ( ! $same and
         ( $x =~ /$numrgx/ ) and
         ( $y =~ /$numrgx/ )) {
        $same = ( $x == $y );
    }
    my $cmp = ( $same ) ? "is the same as" : "differs from";

    print "$x $cmp $y\n";
}
[download]

With that, you can put a pair like "0.123e4 1.230e3" on the command line, and find out that these are the same; and if you do a pair like "0.123e45 1.230e44", these are different. (final update/fix was to add "?" after the first period in $numrgx)

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