I think (but hope wiser heads will correct me if I err) that zwon's case, above rests on the use of non-interpolating quotes around \$number.

Compare to this demo (which can be written much more compactly but was deliberately written in this tedious manner for utmost clarity):

#!/usr/bin/perl -lw
use strict;

my $number0 = '1.00';    #zwon's version with NON-interpolating quotes
my $number1 = 1.00;
my $number2 = 1.393939393939393939393939393939393939393939393939393939
+3939393939;     # exceeds length for FP precision
my $number3 = 123456789.123e5;
my $number4 = 123e-5;
my $number5 = '123e-5'
;
print "\$number0, $number0, contains a period" if $number0 =~ /\./;
print "\$number0, $number0, is an Integer" if $number0 == int($number0
+);

print "\$number1, $number1, contains a period" if $number1 =~ /\./;
print "\$number1, $number1, is an Integer" if $number1 == int($number1
+);

print "\$number2, $number2, contains a period" if $number2 =~ /\./;
print "\$number2, $number2, is an Integer" if $number2 == int($number2
+);

print "\$number3, $number3, contains a period" if $number3 =~ /\./;
print "\$number3, $number3, is an Integer" if $number3 == int($number3
+);

if ( $number4 =~ /\./ ) {       # alt coding of the previous solely to
+ allow the else
    print "\$number4, $number4, contains a period";
} else {
    print "\tNo period in $number4";
}
print "\$number4, $number4, is an Integer" if $number4 == int($number4
+);

if ( $number5 =~ /\./ ) {
    print "\$number5, $number5, contains a period";
} else {
    print "\tNo period in $number5";
}
print "\$number5, $number5, is an Integer" if $number5 == int($number5
+);

print "\nNext, using ww's regex";
print "\n\$number0: $number0\n\$number1: $number1\n\$number2: $number2
+\n\$number3: $number3\n\$number4: $number4\n";

# CASE 0

$number0 =~ /\d+(\.)\d+/;
if ($1) {
    print "\$number0 contains a decimal point";
}
$number0 =~ s/(\d+)(\.)(\d+)/$1/;
print "Integer value, if desired: $number0";

# CASE 1

$number1 =~ /\d+(\.)\d+/;
if ($1) {
    print "\$number1 contains a decimal point";
}
$number1 =~ s/(\d+)(\.)(\d+)/$1/;
print "Integer value of \$number1, if desired: $number1";

# CASE 2

$number2 =~ /\d+(\.)\d+/;
if ($1) {
    print "\$number2 contains a decimal point";
}
$number2 =~ s/(\d+)(\.)(\d+)/$1/;
print "Integer value of \$number2, if desired: $number2";

# CASE 3

$number3 =~ /\d+(\.)\d+/;
if ($1) {
    print "\$number3 contains a decimal point";
}
$number3 =~ s/(\d+)(\.)(\d+)/$1/;
print "Integer value of \$number3, if desired: $number3";

# CASE 4

$number4 =~ /(\.)/;          # Special case: Scientific notation
if ($1) {
    print "\$number4 contains a decimal point";
}
$number4 =~ s/(\d+)(\.)(\d+)/$1/;
print "Integer  value of \$number4, if desired: $number4";

# CASE 5

$number5 =~ /(\.)/;          # Special case, as above
if ($1) {
    print "\$number5 contains a decimal point";
} else {
    print "NO decimal point in \$number5, $number5";
}
$number5 =~ s/(\d=)(\.)(\d+)/$1/;
print "Integer value of \$number5, if desired: " . int($number5);    #
+ using int(); otherwise prints the string literal
[download]

Output:

$number0, 1.00, contains a period
$number0, 1.00, is an Integer        # --> Note 1
$number1, 1, is an Integer
$number2, 1.39393939393939, contains a period
$number3, 12345678912300, is an Integer
$number4, 0.00123, contains a period
        No period in 123e-5

Next, using ww's regex

$number0: 1.00
$number1: 1
$number2: 1.39393939393939
$number3: 12345678912300
$number4: 0.00123

$number0 contains a decimal point
Integer value, if desired: 1
$number1 contains a decimal point
Integer value of $number1, if desired: 1
$number2 contains a decimal point
Integer value of $number2, if desired: 1
$number3 contains a decimal point
Integer value of $number3, if desired: 12345678912300
$number4 contains a decimal point
Integer  value of $number4, if desired: 0
NO decimal point in $number5, 123e-5
Integer value of $number5, if desired: 0
[download]

--> Note 1: Arguable. Integer is defined "n : any of the natural numbers (positive or negative) or zero syn: whole number" and "whole number" is defined "(Math.), a number which is not a fraction or mixed number; an integer."

Update: revised phrasing of the parenthetical comment about the tedious (kindergarten) code.

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