C:\>perl -E "my $gap = 55; $gap = "$gap.00";  say $gap;"
550
C:\>perl -E "my $gap =\"55\"; $gap = "$gap.00";  say $gap;"
550
[download]

Strongly suggest you explain or correct your latest statement.

Suggest you tell us your version of Perl (and show compact version of actual code, as suggested above).

Interesting, slightly older Perl here and it does exactly what the OP indicates. Whether or not it accomplishes what he wants to accomplish is a different subject.

I am baffled that anything would autonomously constrain itself to integer arithmetic -- I think we need to see some code to really assess what is going on.

Update: As a complete side note, the version of Perl doesn't appear to cause a difference in the handling of the string interpretation. I suspect the difference between ww's results and mine are how we chose to display the values (either say vs. print, or command line vs. script file {perhaps the way the quotation mark characters are interpreted from the command line?} ):

#!/usr/bin/perl -w

use strict;

{
    my $originalValue = 5;
    my $modifiedValue = "$originalValue.00";
    my $escapedValue = "$originalValue\.00";

    print "\$originalvalue = '$originalValue'\n";
    print "\$modifiedvalue = '$modifiedValue'\n";
    print "\$escapedvalue = '$escapedValue'\n";
}

exit;

__END__

---------------[ Older Perl ]---------------
C:\Steve\Dev\PerlMonks\P-2013-09-23@1207-ForceFloat>perl -v

This is perl, v5.8.9 built for MSWin32-x64-multi-thread
(with 12 registered patches, see perl -V for more detail)

C:\Steve\Dev\PerlMonks\P-2013-09-23@1207-ForceFloat>forcefloat.pl
$originalvalue = '5'
$modifiedvalue = '5.00'
$escapedvalue = '5.00'

---------------[ Newer Perl ]---------------
C:\Steve\Dev\PerlMonks\P-2013-09-23@1207-ForceFloat>perl -v

This is perl 5, version 16, subversion 3 (v5.16.3) built for MSWin32-x
+64-multi-thread
(with 1 registered patch, see perl -V for more detail)

C:\Steve\Dev\PerlMonks\P-2013-09-23@1207-ForceFloat>forcefloat.pl
$originalvalue = '5'
$modifiedvalue = '5.00'
$escapedvalue = '5.00'
[download]

I think I've found the culprit, but I will be providing code this evening (USA East Coast time). This is part of a big project (bookkeeping, not programming) that I'm on a deadline on. I put together a sloppy and quick test program I'll tidy up for posting here. It seems the issue is one module used Math::BigInt. I'll include new information when I add the code tonight.

I noticed you backslashed some of your quotes but not others. Using qq() may help:

C:\>perl -E "my $gap = 55; $gap = qq[$gap.00]; say $gap;"
55.00
[download]