#!/usr/bin/perl
use warnings;
use strict;
use 5.010;

my $string = 'SRC=array.c builtin.c \
missing.c msg.c';

#Regex-1 Following matches, but WHY? String has a single space between "builtin.c \"
$string =~ m/^\w+=[^\n\\]*\\/;

#Regex-2 Following also matches
$string =~ m/^\w+=[^\n\\]* \\/; #pay attention to single space between * and \

#Regex-3 Following does NOT match
$string =~ m/^\w+=[^\n\\]*  \\/; #pay attention to double spaces between * and \

say $string;