As davido and BUU referred to here's an example...
If your input file isn't too big you could do something like this...

#!/usr/bin/perl -W

use strict;

my $string = <<HERE;
abc defg
hijk    lm
no  pqr
stu
vwkyz
HERE
 
my $find = "hijklmnop";

# get a copy to avoid modifying orig
$_ = $string;

# remove all spaces 
s/\s//gs;  # note the /s that davido referred to
 
print "Matched!\n" if $_ =~ $find;

# to see what the subst did.
print $_;
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Not sure which is more efficient though...

Also, it seems like you could do something like this, never modifying the orig string...
But I couldn't get it to work. May the gurus can help...

my $result;
$_ = $string

($result) = /((\w+(?=\s*))+)/s;
# ((1 char) followed by 
# (zero or more whitespace[don't return])
# match one or more)
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