Perl中的qrSTRING运算符如何决定是否编译STRING

Perl中的qr/STRING/运算符如何决定是否编译STRING？

use strict;
use warnings;
my $string = "Hello, World!";
my $pattern = qr/World/;
if ($string =~ $pattern) {
    print "Match found!\n";
} else {
    print "No match found!\n";
}

use strict;
use warnings;
# 字符串中不包含变量、特殊字符或表达式
my $pattern1 = qr/abc/;  # 立即编译
# 字符串中包含变量、特殊字符或表达式
my $var = "def";
my $pattern2 = qr/abc$var/;  # 延迟编译
# 字符串中包含无法解析的变量、特殊字符或表达式
my $pattern3 = qr/abc$unknown_var/;  # 编译错误
# 使用正则表达式对象进行匹配操作
my $string = "abcdef";
if ($string =~ $pattern1) {
    print "Pattern 1 matched!\n";
}
if ($string =~ $pattern2) {
    print "Pattern 2 matched!\n";
}