x	- so that you can follow similar formatting/readabiliy guidlines as presented ealier in the book.	PBP pg 236
m	- so that ^ and $ behave like most people expect (e.g. like in grep, sed, awk).	PBP pg 237
s	- so that . matches anything like most people expect.	PBP pg 240

For example, which is easier to read?

my $pattern = qr{HOSTNAME:\s*(?:\d{4}-)?\Q$hostname_for_regex\E(grbr-r
+wan-x)\s+GRBR:\s*\Q$dev_ref->{grbr}\E}ixms;
[download]

my $pattern = qr{            # quoted regex
    HOSTNAME:                # Literal text
    \s*                      # 0 or more whitespace
    (?:\d{4}-)?              # Some devices begin with the router mode
+l
    \Q$hostname_for_regex\E  # Router hostname (grbr-rwan-x)
    \s+                      # 1 or more whitespace
    GRBR:                    # Literal text
    \s*                      # 0 or more whitespace
    \Q$dev_ref->{grbr}\E     # Just the GRBR
    }ixms;
[download]