use warnings;
use strict;

my @wanted_substrings;

{
    local $/ = 'END';

    while(my $string = <DATA>) {
        push @wanted_substrings, ($string =~ /START(.*)END/);
    }

}

print "@wanted_substrings";
    

__DATA__
somecharSTARTfvENDsomecharSTARTsvENDsomecharSTARTtvENDsomechar