You need to declare variables when you have use strict; see strict. Also when reading from they keyboard you need to use chomp:

#!/usr/bin/perl
use strict;
use warnings;

print "How old are you?\n";
chomp (my $age = <>);
print "What is your favorite color?\n";
chomp (my $color = <>);
print "You are $age, and your favorite color is $color.\n";
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Note that the second example is using chomp already, but missing declorations:

#!/usr/bin/perl
use strict;
use warnings;

print "What is the radius of the circle?\n";
chomp (my $r = <>);

my $diameter = (2 * $r);
my $area = (3.14 * ($r ** 2));
my $cir = ($diameter * 3.14);
print "Radius: $r\n Diameter: $diameter\n Circumference: $cir\nArea: $
+area\n";
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References for learning Perl:

• perlintro
• http://learn.perl.org
• http://perldoc.perl.org
• The tutorials section of this site