To answer your direct question, you backslash them. The string literal «"foo \\ bar"» produces the string «foo \ bar».

But that's the least of your problems, /.../ is the match operator, so it's Perl code. If you want to compile and run Perl code, you'd have to use eval EXPR.

But if @search contained regular expressions instead of Perl code, you could wouldn't need to go to such drastic measures.

my @patterns = (
   "<ID>(.*?)</ID>",                      # ID
   "<TimeStamp>(.*?)</TimeStamp>",        # Time Stamp
   "<IP_Address>(.*?)</IP_Address>",      # IP Address
   "<Title>(.*?)</Title>",                # Title
   "<Complainant><Entity>(.*?)</Entity>", # Reporting Entity
   "<Contact>(.*?)</Contact>",            # Reporting Entity Contact
   "<Address>(.*?)</Address>",            # Reporting Entity Address
   "</Phone><Email>(.*?)</Email>",        # Reporting Entity Email
);

my @xml_files = <*xml>;
for my $file (@xml_files) {
   for my $pattern (@patterns) {
      ...
      if ($line =~ $pat) {
      ...
   }
}
[download]

Furthermore, it would be beneficial to use qr//.

my @regexps = (
   qr{<ID>(.*?)</ID>},                      # ID
   qr{<TimeStamp>(.*?)</TimeStamp>},        # Time Stamp
   qr{<IP_Address>(.*?)</IP_Address>},      # IP Address
   qr{<Title>(.*?)</Title>},                # Title
   qr{<Complainant><Entity>(.*?)</Entity>}, # Reporting Entity
   qr{<Contact>(.*?)</Contact>},            # Reporting Entity Contact
   qr{<Address>(.*?)</Address>},            # Reporting Entity Address
   qr{</Phone><Email>(.*?)</Email>},        # Reporting Entity Email
);

my @xml_files = <*xml>;
for my $file (@xml_files) {
   for my $regexp (@regexps) {
      ...
      if ($line =~ $regexp) {
      ...
   }
}
[download]