I only want the escape character to be treated specially if it's in !+;

Yuck! I hope you're being forced to deal with this format.

It's not only tricker for a human to understand, it's tricker to code. In particular, the definition of a field varies based on whether it's the last field or not, and the function of the "!" varies based on its position in the field.

sub unescape {
    my $x = $_[0];
    my ($base, $end) = $x =~ /^(.*)(!+)\z/s;
    return $base . ('!' x (length($end)/2));
}

my $last_field = qr/
   [^;]*
/x;

my $other_field = qr/
   (?: [^!]+
   |   (?: ![^!] )+
   )*
   (?:!!)*
/x;

# Validation
my $record = qr/^ (?: $other_field ; )* $last_field \z/x;

# Extraction
my @fields = map unescape($_), /
   \G
   ( $other_field (?= ; )
   | $last_field  (?= \z )
   )
   (?:;|\z)
/xg;
[download]

You are free to skip the validation.