That doesn't work. Since you don't capture .*, your look-behind is identical to m/tom.*(dick|harry|john)/. You also can't put a regexp inside a look-behind, IIRC.

'  tom  harry  ' =~ m/(?<=tom).*(dick|harry|john)/ && print(1, $/);  #
+ 1
'  harry  tom  ' =~ m/(?<=tom).*(dick|harry|john)/ && print(2, $/);  #
+ (nothing)
[download]

Lookahead works, though

'  tom  harry  ' =~ m/(?=.*tom).*(dick|harry|john)/ && print('A', $/);
+  # A
'  harry  tom  ' =~ m/(?=.*tom).*(dick|harry|john)/ && print('B', $/);
+  # B
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