$def =~ s/
    (^\.\w+\s)      ## $1 := A '.', followed a word and a space
    (\w+)           ## $2 := Another word
    (\s+)           ## $3 := some whitespace
    (.+)            ## $4 := Some anything
    (               ## $5
        ^           ## The start of string/line
        select      ## The word 'select'
        \.          ## A literal '.'
        .           ## Any single character
        $           ## The end of the line/string
    )
/$2$5/x;        ## put back just first word and the 'select' etc

##</code><code>##

#! perl 
use warnings;
use strict;

my $def = <<'EOD';
.define get_user_info
        select  user_location,user_name, user_company, &fax_header || ';style=' || letterhead,
                fax_printer, nvl(fax_yn,'N')
        into    user_location, user_name, user_company, fax_header, fax_printer, fax_yn
        from    security_header
        where   user_id = lower(substr(user,5,10))
..
EOD

$def =~ s/
    ( \A \. \w+ \s ) ## $1 := A '.', followed a word and a space
    (\w+)           ## $2 := Another word
    (\s+)           ## $3 := Some whitespace
    (.+?)           ## $4 := Some anything
    (               ## $5
        \s+         ##  Some whitespace
        select      ##  The word 'select'
        [^.]+       ##  Lots of anything except a '.'
        \. \. \Z    ##  Two '.'s and the end of line
    )
/$2$5/smx        ## . matches \n; ^ & $ match lines with the string
or die 'Failed to match';

print $def;