Consider the following:

my $value = 100;

my $string = "$value";

print $string, "\n";
__OUTPUT__
100
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Ok, here's the problem. What you just witnessed is $value being interpolated as a variable into a string, and assigned to $string. Seems harmless. But it can easily get you into trouble.

my $var = "Hello world!";
my $evalcode = "print $var, qq/\n/;";
eval $evalcode or print "There was a problem:\n$@\n";
__OUTPUT__
There was a problem:
syntax error at (eval 1) line 1, near "!,"
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The problem was that before eval got a chance to see $var, it got interpolated by the double quotes and thus was seen by eval as a literal string rather than a variable. The code that eval saw was:
print Hello world, qq/\n/;

See the error? You wanted eval to see
print $var, qq/\n/;. Instead, you got an unquoted literal string that looked a lot like a bunch of garbage to Perl.

The situation can be remedied by either using single quotes, or by escaping your double-quoted variables so that eval gets them uninterpolated. It can be tricky deciding which option to use. ...but that's the nature of eval... a little tricky. ;)