Just for the fun of it (and a gentle push towards "use a templating system"), I came up with this:

use strict;
use warnings;
use HTML::Template;
my $t = HTML::Template->new( 
    filehandle => \*DATA, 
    die_on_bad_params => 0 );
my ( $average, @fullstars, @emptystars );
$average = 3.7; # hard-coded for testing
for ( 1 .. 5 ) {
    if ( $_ <= $average ) {
        push( @fullstars, 1 );
    }
    else { push( @emptystars, 1 ) }
}

if ( int($average) != $average ) {
    $t->param( halfstar => 1 );
    shift(@emptystars);
}
$t->param(
    fullstars  => \@fullstars,
    emptystars => \@emptystars
);
print $t->output();

__DATA__
<tmpl_loop name="fullstars">
    <img src="fullstar">
</tmpl_loop>
<tmpl_if name="halfstar">
    <img src="halfstar">
</tmpl_if>
<tmpl_loop name="emptystars">
    <img src="emptystar">
</tmpl_loop>
[download]