... "X-ray" simply fails.

What others have written remains true: a '-' (dash; hyphen) character is not the same as (and will not match) a ' ' (space) character. (Also pay attention to hdb's reply below (update: among others) concerning case-insensitive matching.) Consider these matches to get a feel for regex matching:

c:\@Work\Perl\monks>perl -wMstrict -le
"my @X = ('aX-raya', 'aX raya', 'aXraya', 'axraya', 'X Ray', 'aXYraya'
+);
 ;;
 print 'case-sensitive matching:';
 for my $s (@X) {
   printf qq{'$s' -> };
   if ($s =~ m{ X [- ]? ray }xms) {
     print 'match';
     }
   else {
     print 'NO match';
     }
   }
 ;;
 print 'case-INsensitive matching:';
 for my $s (@X) {
   printf qq{'$s' -> };
   if ($s =~ m{ (?i) X [- ]? ray }xms) {
     print 'match';
     }
   else {
     print 'NO match';
     }
   }
"
case-sensitive matching:
'aX-raya' -> match
'aX raya' -> match
'aXraya' -> match
'axraya' -> NO match
'X Ray' -> NO match
'aXYraya' -> NO match
case-INsensitive matching:
'aX-raya' -> match
'aX raya' -> match
'aXraya' -> match
'axraya' -> match
'X Ray' -> match
'aXYraya' -> NO match
[download]

Contrast the use of  [- ]? with  . (dot) as a placeholder to match anything at all:

c:\@Work\Perl\monks>perl -wMstrict -le
"my @X = ('aX-raya', 'aX raya', 'aXraya', 'axraya', 'X Ray', 'aXYraya'
+);
 ;;
 print 'placeholder matching:';
 for my $s (@X) {
   printf qq{'$s' -> };
   if ($s =~ m{ X . ray }xms) {
     print 'match';
     }
   else {
     print 'NO match';
     }
   }
"
placeholder matching:
'aX-raya' -> match
'aX raya' -> match
'aXraya' -> NO match
'axraya' -> NO match
'X Ray' -> NO match
'aXYraya' -> match
[download]

In general, it's better to match exactly what you want.

Please see the Perl regex documentation perlre, perlretut, perlrequick, and various regex tutorials on Perlmonks.